tab_powerline_style of round appears to be broken

[salmankhilji]

Describe the bug
With the tab_powerline_style set to round, the separators do not appear correctly. See attached screenshot.

To Reproduce
Steps to reproduce the behavior:

1. Launch kitty like this:

   kitty -o tab_bar_style=powerline -o tab_powerline_style=round
   

2. Open a few tabs.

Expected behavior
The powerline separators shall be round.

Observed behavior
The powerline separators show as placeholders.

Screenshots

Environment details
OS: Name and version of operating system(s)

Ubuntu 20.04 LTS

$ kitty --debug-config
kitty 0.20.1 (62656b24eb) created by Kovid Goyal
Linux byteevo-scm 5.8.0-50-generic #56~20.04.1-Ubuntu SMP Mon Apr 12 21:46:35 UTC 2021 x86_64
Ubuntu 20.04.2 LTS \n \l
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.2 LTS"
Loaded config files: /home/precor/.config/kitty/kitty.conf
Running under: X11

Config options different from defaults:
adjust_line_height    1
font_family           Hack
initial_window_height (32, 'cells')
initial_window_width  (128, 'cells')
remember_window_size  False
tab_bar_style         powerline
Added shortcuts:
	ctrl+l KeyAction(func='combine', args=(KeyAction(func='clear_terminal', args=['scrollback', True]), KeyAction(func='send_text', args=['normal,application', b'\x0c'])))
	ctrl+tab KeyAction(func='next_window', args=())
	ctrl+page_up KeyAction(func='previous_tab', args=())
	ctrl+page_down KeyAction(func='next_tab', args=())
	shift+ctrl+tab KeyAction(func='previous_window', args=())
Changed shortcuts:
	shift+ctrl+t KeyAction(func='launch', args=['--type=tab', '--cwd=current'])

Additional context

• kitty is compiled from source. master branch at 62656b24.