Investigated as part of
https://github.com/flutter/flutter/issues/127232.
This took some time to work out, but:
* The amount of blending applied to the source color needs to be
weighted by the destination alpha, which is pretty sensible behavior.
This is in addition to applying source-over behavior with the resulting
blended color.
* All of the blend functions assume that the color is already
premultiplied, so remove the unpremultiply/premultiply surrounding the
blend call.
All of the blend modes now visually match up with the [Flutter
docs](https://api.flutter.dev/flutter/dart-ui/BlendMode.html), except
for ColorBurn and Saturation, which appear to have a slight
miscalculation going on with the red channel.
09246795b0