Strikethrough is broken after 0.21.0

[ahmedelgabri]

Describe the bug
Strikethrough is not rendered correctly in 0.21.0, was working before.

To Reproduce

echo -e "\e[9mstrikethrough\e[0m"

Screenshots
kitty 0.21.0 with Kitty --config NONE

Environment details

kitty 0.21.0 created by Kovid Goyal
Darwin pandoras-box 20.5.0 Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64 x86_64
ProductName:	macOS ProductVersion:	11.4 BuildVersion:	20F71
Loaded config files:
  /Users/ahmed/.config/kitty/kitty.conf

Config options different from defaults:
active_border_color          None
allow_remote_control         y
background                   Color(red=17, green=17, blue=17)
bold_font                    PragmataPro Mono Liga Bold
bold_italic_font             PragmataPro Mono Liga Bold Italic
clipboard_control            ('write-clipboard', 'write-primary', 'no-append')
cursor                       Color(red=32, green=187, blue=252)
cursor_stop_blinking_after   0
enable_audio_bell            False
font_family                  PragmataPro Mono Liga
font_features:
{'PragmataProMonoLiga-Regular': ('+ss13',)}
font_size                    12.0
force_ltr                    True
foreground                   Color(red=197, green=200, blue=198)
hide_window_decorations      1
inactive_tab_background      Color(red=29, green=31, blue=33)
italic_font                  PragmataPro Mono Liga Italic
kitty_mod                    8
macos_custom_beam_cursor     True
macos_option_as_alt          3
macos_thicken_font           1.1
macos_traditional_fullscreen True
scrollback_lines             20000
select_by_word_characters    :@-./_~?&=%+#
strip_trailing_spaces        smart
tab_activity_symbol          ▲
tab_bar_edge                 1
window_alert_on_bell         False
window_margin_width          FloatEdges(left=10.0, top=10.0, right=10.0, bottom=10.0)
window_padding_width         FloatEdges(left=10.0, top=10.0, right=10.0, bottom=10.0)
Added shortcuts:
	ctrl+i KeyAction('send_text', ('application', b'H'))
	cmd+. KeyAction('move_tab_forward')
	cmd+[ KeyAction('previous_window')
	cmd+] KeyAction('next_window')
	cmd+` KeyAction('move_window_to_top')
	cmd+a > 1 KeyAction('set_background_opacity', ('1',))
	cmd+a > d KeyAction('set_background_opacity', ('default',))
	cmd+a > l KeyAction('set_background_opacity', ('-0.1',))
	cmd+a > m KeyAction('set_background_opacity', ('+0.1',))
	cmd+b KeyAction('move_window_backward')
	cmd+e KeyAction('kitten', ('hints',))
	cmd+f KeyAction('move_window_forward')
	cmd+h KeyAction('previous_window')
	cmd+j KeyAction('scroll_line_down')
	cmd+k KeyAction('scroll_line_up')
	cmd+l KeyAction('next_layout')
	cmd+o KeyAction('pass_selection_to_program')
	cmd+p > f KeyAction('kitten', ('hints', '--type path --program -'))
	cmd+p > h KeyAction('kitten', ('hints', '--type hash --program -'))
	cmd+p > l KeyAction('kitten', ('hints', '--type line --program -'))
	cmd+p > n KeyAction('kitten', ('hints', '--type linenum'))
	cmd+p > w KeyAction('kitten', ('hints', '--type word --program -'))
	cmd+p > y KeyAction('kitten', ('hints', '--type hyperlink'))
	cmd+p > shift+f KeyAction('kitten', ('hints', '--type path'))
	cmd+q KeyAction('close_tab')
	cmd+s KeyAction('paste_from_selection')
	cmd+u KeyAction('input_unicode_character')
	cmd+escape KeyAction('kitty_shell', ('window',))
	cmd+backspace KeyAction('change_font_size', (True, None, 0.0))
	cmd+delete KeyAction('clear_terminal', ('reset', True))
	cmd+left KeyAction('previous_tab')
	cmd+right KeyAction('next_tab')
	cmd+f2 KeyAction('edit_config_file')
	cmd+f5 KeyAction('load_config_file')
	cmd+f6 KeyAction('debug_config')
	cmd+f10 KeyAction('toggle_maximized')
	cmd+f11 KeyAction('toggle_fullscreen')
	cmd+kp_subtract KeyAction('change_font_size', (True, '-', 2.0))
	cmd+kp_add KeyAction('change_font_size', (True, '+', 2.0))
	shift+cmd+h KeyAction('show_scrollback')
	alt+cmd+t KeyAction('set_tab_title')
Removed shortcuts:
	shift+ctrl++ KeyAction('change_font_size', (True, '+', 2.0))
	shift+ctrl+, KeyAction('move_tab_backward')
	shift+ctrl+- KeyAction('change_font_size', (True, '-', 2.0))
	shift+ctrl+. KeyAction('move_tab_forward')
	shift+ctrl+0 KeyAction('tenth_window')
	shift+ctrl+1 KeyAction('first_window')
	shift+ctrl+2 KeyAction('second_window')
	shift+ctrl+3 KeyAction('third_window')
	shift+ctrl+4 KeyAction('fourth_window')
	shift+ctrl+5 KeyAction('fifth_window')
	shift+ctrl+6 KeyAction('sixth_window')
	shift+ctrl+7 KeyAction('seventh_window')
	shift+ctrl+8 KeyAction('eighth_window')
	shift+ctrl+9 KeyAction('ninth_window')
	shift+ctrl+= KeyAction('change_font_size', (True, '+', 2.0))
	shift+ctrl+[ KeyAction('previous_window')
	shift+ctrl+] KeyAction('next_window')
	shift+ctrl+` KeyAction('move_window_to_top')
	shift+ctrl+a > 1 KeyAction('set_background_opacity', ('1',))
	shift+ctrl+a > d KeyAction('set_background_opacity', ('default',))
	shift+ctrl+a > l KeyAction('set_background_opacity', ('-0.1',))
	shift+ctrl+a > m KeyAction('set_background_opacity', ('+0.1',))
	shift+ctrl+b KeyAction('move_window_backward')
	shift+ctrl+c KeyAction('copy_to_clipboard')
	shift+ctrl+e KeyAction('kitten', ('hints',))
	shift+ctrl+f KeyAction('move_window_forward')
	shift+ctrl+h KeyAction('show_scrollback')
	shift+ctrl+j KeyAction('scroll_line_down')
	shift+ctrl+k KeyAction('scroll_line_up')
	shift+ctrl+l KeyAction('next_layout')
	shift+ctrl+n KeyAction('new_os_window')
	shift+ctrl+o KeyAction('pass_selection_to_program')
	shift+ctrl+p > f KeyAction('kitten', ('hints', '--type path --program -'))
	shift+ctrl+p > h KeyAction('kitten', ('hints', '--type hash --program -'))
	shift+ctrl+p > l KeyAction('kitten', ('hints', '--type line --program -'))
	shift+ctrl+p > n KeyAction('kitten', ('hints', '--type linenum'))
	shift+ctrl+p > w KeyAction('kitten', ('hints', '--type word --program -'))
	shift+ctrl+p > y KeyAction('kitten', ('hints', '--type hyperlink'))
	shift+ctrl+p > shift+f KeyAction('kitten', ('hints', '--type path'))
	shift+ctrl+q KeyAction('close_tab')
	shift+ctrl+r KeyAction('start_resizing_window')
	shift+ctrl+s KeyAction('paste_from_selection')
	shift+ctrl+t KeyAction('new_tab')
	shift+ctrl+u KeyAction('kitten', ('unicode_input',))
	shift+ctrl+v KeyAction('paste_from_clipboard')
	shift+ctrl+w KeyAction('close_window')
	shift+ctrl+escape KeyAction('kitty_shell', ('window',))
	shift+ctrl+enter KeyAction('new_window')
	shift+ctrl+backspace KeyAction('change_font_size', (True, None, 0.0))
	shift+ctrl+delete KeyAction('clear_terminal', ('reset', True))
	shift+ctrl+left KeyAction('previous_tab')
	shift+ctrl+right KeyAction('next_tab')
	shift+ctrl+up KeyAction('scroll_line_up')
	shift+ctrl+down KeyAction('scroll_line_down')
	shift+ctrl+page_up KeyAction('scroll_page_up')
	shift+ctrl+page_down KeyAction('scroll_page_down')
	shift+ctrl+home KeyAction('scroll_home')
	shift+ctrl+end KeyAction('scroll_end')
	shift+ctrl+f2 KeyAction('edit_config_file')
	shift+ctrl+f5 KeyAction('load_config_file')
	shift+ctrl+f6 KeyAction('debug_config')
	shift+ctrl+f10 KeyAction('toggle_maximized')
	shift+ctrl+f11 KeyAction('toggle_fullscreen')
	shift+ctrl+kp_subtract KeyAction('change_font_size', (True, '-', 2.0))
	shift+ctrl+kp_add KeyAction('change_font_size', (True, '+', 2.0))
	shift+alt+ctrl+t KeyAction('set_tab_title')
Changed shortcuts:
	cmd+, KeyAction('move_tab_backward')
	cmd+w KeyAction('close_window')

Additional context
Reproducible with Kitty --config NONE