An additional layer is required to composite correctly. PaintComposite
blends/composites between source and destination. The result is
then drawn on top of previous paint results. Previously, the source
layer was just drawn atop previous paints, then the destination
layer was composited down with composite mode, which was incorrect.
More details in [1].
Add tests for composite modes, compare [2]
[1] https://github.com/googlefonts/nanoemoji/issues/332
[2] https://github.com/googlefonts/color-fonts/pull/73
Cq-Include-Trybots: luci.skia.skia.primary:Test-Android-Clang-GalaxyS6-GPU-MaliT760-arm64-Release-All-Android_NativeFonts,FM-Debian10-Clang-GCE-CPU-AVX2-x86_64-Release-All-MSAN
Bug: skia:12424
Change-Id: I8ff69022ec09758af7021756b0fb26443216fcca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/445936
Commit-Queue: Dominik Röttsches <drott@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
09961b554d