move dither in SkVMBlitter
move dither in SkVMBlitter

This moves dither up to the end of the shader pipeline, rather than
being part of the fixed pipeline after blend and coverage resolve.

This is the easiest, lowest-consequence way to experiment with this.

Very few, tiny diffs.  Most of what we draw dithered is in src-mode
(generally opaque srcover) with full coverage and has mathematically
not changed.

Bug: skia:10000
Change-Id: I2df3ab1d15790ed57ff9014de568b1d1a717a387
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/275337
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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