[Impeller] Investigate if glyphs should be cached with a canonicalized color

[jonahwilliams]

Skia uses https://github.com/google/skia/blob/f8a6c43e67847902aa3779482ca78e7beb88ebe4/src/text/gpu/TextBlob.cpp#L60 to compute a canonical color for caching glyphs. The color the text is drawn with has an implact on the anti aliasing pixels.

Previously we made a tradeoff to make dark text on light backgrounds look better, potentially at the cost of light text on dark backgrounds.

[jonahwilliams]

Actually, we may not need to worry about this - isntead using an A8 or R8 may just fix this since we can treat the glyph as coverage data and not worry about color info.

[github-actions]

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