Add comment explaining unpremul paint color handling
Add comment explaining unpremul paint color handling

Change-Id: Ibe056629a78c49f9d03eb7658c605866f31fafe5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/467303
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Auto-Submit: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>

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