[graphite] Add color conversion support to asyncReadPixels.

Uses ConvertPixels as a post-process step.
Also updated the asyncRead tests to check for proper conversion.

Bug: b/238756873
Change-Id: I001d9428206e1cfa43dbba506fcca6cc9f171f68
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/598238
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed