scrollback_indicator_opacity is binary / does not vary opacity

[ethanwu10]

Describe the bug
When setting scrollback_indicator_opacity, it appears to be a binary on/off setting instead of actually controlling opacity, in that when it is set to anything >0, the scrollback indicator is the same opacity regardless of the exact value of scrollback_indicator_opacity

I noticed in the shader code, the alpha is hardcoded 0.8, and the config option is never used except to see if it is <= 0 (and disable the shader entirely).

kitty/kitty/shaders.c

Line 615 in fbdc2b4

GLfloat alpha = 0.8f;

To Reproduce
Steps to reproduce the behavior:

1. Set scrollback_indicator_opacity to something extremely low like 0.001
2. Notice the indicator is just as visible as the default (1.0)

Environment details

kitty 0.35.1 (1bfb1de7fc) created by Kovid Goyal
Darwin macbook 23.5.0 Darwin Kernel Version 23.5.0: Wed May  1 20:14:38 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6020 arm64
ProductName:		macOS ProductVersion:		14.5 BuildVersion:		23F79
OpenGL: '4.1 Metal - 88.1' Detected version: 4.1
Frozen: True
Paths:
  kitty: /Applications/kitty.app/Contents/MacOS/kitty
  base dir: /Applications/kitty.app/Contents/Resources/kitty
  extensions dir: /Applications/kitty.app/Contents/Resources/Python/lib/kitty-extensions
  system shell: /bin/zsh
Loaded config files:
  /Users/user/.config/kitty/kitty.conf

Config options different from defaults:
font_family                  Hack
initial_window_height        (25, 'cells')
initial_window_width         (80, 'cells')
macos_option_as_alt          3
macos_show_window_title_in   window
mouse_hide_wait              3.0
remember_window_size         False
scrollback_indicator_opacity 0.001
symbol_map:
	U+23fb - U+23fe → Symbols Nerd Font Mono
	U+2665 - U+2665 → Symbols Nerd Font Mono
	U+26a1 - U+26a1 → Symbols Nerd Font Mono
	U+2b58 - U+2b58 → Symbols Nerd Font Mono
	U+e000 - U+e00a → Symbols Nerd Font Mono
	U+e0a0 - U+e0a2 → Symbols Nerd Font Mono
	U+e0a3 - U+e0a3 → Symbols Nerd Font Mono
	U+e0b0 - U+e0b3 → Symbols Nerd Font Mono
	U+e0b4 - U+e0c8 → Symbols Nerd Font Mono
	U+e0ca - U+e0ca → Symbols Nerd Font Mono
	U+e0cc - U+e0d4 → Symbols Nerd Font Mono
	U+e200 - U+e2a9 → Symbols Nerd Font Mono
	U+e300 - U+e3e3 → Symbols Nerd Font Mono
	U+e5fa - U+e6b1 → Symbols Nerd Font Mono
	U+e700 - U+e7c5 → Symbols Nerd Font Mono
	U+ea60 - U+ebeb → Symbols Nerd Font Mono
	U+f000 - U+f2e0 → Symbols Nerd Font Mono
	U+f300 - U+f372 → Symbols Nerd Font Mono
	U+f400 - U+f532 → Symbols Nerd Font Mono
	U+f0001 - U+f1af0 → Symbols Nerd Font Mono
window_margin_width          FloatEdges(left=5.0, top=5.0, right=5.0, bottom=5.0)
Changed mouse actions:
	left click ungrabbed →  mouse_handle_click selection prompt
Removed shortcuts:
	cmd+, →  edit_config_file
	cmd+enter →  new_window
	cmd+h →  hide_macos_app
	cmd+k →  clear_terminal to_cursor active
	cmd+m →  minimize_macos_window
	cmd+n →  new_os_window
	cmd+q →  quit
	cmd+t →  new_tab
	cmd+w →  close_tab
	ctrl+cmd+, →  load_config_file
	ctrl+cmd+f →  toggle_fullscreen
	opt+cmd+h →  hide_macos_other_apps
	opt+cmd+r →  clear_terminal reset active
	opt+cmd+s →  toggle_macos_secure_keyboard_entry
	shift+cmd+/ →  open_url https://sw.kovidgoyal.net/kitty/
	shift+cmd+[ →  previous_tab
	shift+cmd+] →  next_tab
	shift+cmd+d →  close_window
	shift+cmd+w →  close_os_window
Changed shortcuts:
	kitty_mod+page_down →  next_tab
	kitty_mod+page_up →  previous_tab
Colors:
	active_border_color          #65737e   
	active_tab_background        #1b2b34   
	active_tab_foreground        #c0c5ce   
	background                   #1b2b34   
	color0                       #1b2b34   
	color1                       #ec5f67   
	color10                      #99c794   
	color11                      #fac863   
	color12                      #6699cc   
	color13                      #c594c5   
	color14                      #5fb3b3   
	color15                      #d8dee9   
	color16                      #f99157   
	color17                      #ab7967   
	color18                      #343d46   
	color19                      #4f5b66   
	color2                       #99c794   
	color20                      #a7adba   
	color21                      #cdd3de   
	color3                       #fac863   
	color4                       #6699cc   
	color5                       #c594c5   
	color6                       #5fb3b3   
	color7                       #c0c5ce   
	color8                       #65737e   
	color9                       #ec5f67   
	cursor                       #c0c5ce   
	foreground                   #c0c5ce   
	inactive_border_color        #343d46   
	inactive_tab_background      #343d46   
	inactive_tab_foreground      #a7adba   
	selection_background         #c0c5ce   
	selection_foreground         #1b2b34   
	tab_bar_background           #343d46   
	url_color                    #a7adba   

Important environment variables seen by the kitty process:
	PATH                                /Applications/kitty.app/Contents/MacOS:/usr/bin:/bin:/usr/sbin:/sbin
	LANG                                en_US.UTF-8
	SHELL                               /bin/zsh
	DISPLAY                             /private/tmp/com.apple.launchd.o70pjlQINQ/org.xquartz:0
	USER                                user