Add documentation on blitting glyphs.
Add documentation on blitting glyphs.

Document why mask gamma, optical sizing, and contrast hacks are used and
where. In addition to documentation this also justifies where contrast
settings should be placed in the API.

Change-Id: I8c67d800a95e62360a88c815d3224e0c05ed65c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356305
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia