Add always dither SkSurfaceProps flag

Allows an SkSurface to be configured to always dither
such as when rendering to restricted bit depths for
the desired colorspace

Bug: skia:14238

Change-Id: I7ae6ff1f62daf8ce2aee825401c0f33df0f46bf2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/666860
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Reck <jreck@google.com>
3 files changed