gs-feature-tile: Switch to WCAG contrast instead of Weber contrast
The Weber contrast was routinely always returning -1.0
for dark
foreground colours (i.e. black text), which meant it was not so useful
for making decisions with.
Switching to the WCAG contrast ratio (as defined at https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html) seems to produce better results.
Signed-off-by: Philip Withnall pwithnall@endlessos.org
Fixes: #1440 (closed)
Closes #1440 (closed)