fonts: Remove web fonts when their stylsheet is removed
This is the first part of ensuring that unused fonts do not leak. This
change makes it so that when a stylesheet is removed, the corresponding
web fonts are removed from the FontContext.
Note: WebRender assets are still leaked, which was the situation before
for all fonts. A followup change will fix this issue.
Fixes #15139.
Signed-off-by: Martin Robinson mrobinson@igalia.com
Co-authored-by: Mukilan Thiyagarajan mukilan@igalia.com