text_composition_strategy legacy is noticeably worse than pre-0.28 rendering #6209

Describe the bug

Relating to Kitty 0.28 text_composition_strategy legacy:

1. it's always subtly different from actual pre-0.28 rendering, and
2. it looks much worse with transparent windows in front of light backgrounds.

To Reproduce
Steps to reproduce the behavior:

1. Use config as shown below
2. Update to kitty 0.28
3. Observe text (as shown in screenshots)

Screenshots

Note: Screenshots were taken in a kitty instance with full transparency — the background color is exclusively from blur (via my compositor) over my desktop background.

Post-0.28

Pre-0.28

Note: With the darker background, there is also a slight change that post-0.28 rendering is slightly thicker - should hopefully be easy enough to see if you compare the two.

Environment details

Couldn't get Ctrl+Shift+F6 working, for some reason. Pasting the relevant pieces of my config file. kitty -v outputs kitty 0.28.1 created by Kovid Goyal. Happy to provide more details upon request.

### Fonts ###
font_family Ubuntu Mono
font_size   20.0

disable_ligatures always
text_composition_strategy legacy

### Color scheme ###
foreground #eaeaea
background #263238

# Terminal colors
#
# These are given first with the normal variety, and then with the
# bold variety.
#
# Black
color0 #000000
color8 #666666
# Red
color1 #d54e53
color9 #ff3334
# Green
color2  #b9ca4a
color10 #9ec400
# Yellow
color3  #e6c547
color11 #e6c547
# Blue
color4  #7aa6da
color12 #7aa6da
# Magenta
color5  #c397d8
color13 #b77ee0
# Cyan
color6  #70c0ba
color14 #54ced6
# White
color7 #eaeaea
color15 #ffffff

### Advanced options ###
background_opacity 0.0

# note: lines unrelated to rendering were omitted.

I think the font color in the screenshots is color4 (normal blue), but not 100% sure.