Bug 1912431 - Don't apply global alpha when creating a cached color glyph. a=RyanVM

The reftest here currently fails on Android for what appears to be an
unrelated reason: reftest-analyzer shows that in the *reference* case,
all but the last line of text is completely missing.

I'm not sure what's failing there (but it fails similarly without this
patch, anyhow); we should investigate that as a separate bug.

Original Revision: https://phabricator.services.mozilla.com/D219024

Differential Revision: https://phabricator.services.mozilla.com/D219886