GrConvertPixels takes pixmaps

Add GrCPixmap, a GrPixmap but with const void* instead of void*. Share
impl via template base class GrPixmapBase.

Change-Id: I7dfdf24a73c1bc8557ff7b90f93a9399da2f3f75
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/350022
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
25 files changed