[graphite] AsyncReadPixels clean up.
* Removed unnecessary const_cast
* Surfaces can only be created with kPremul alphaType.
* Now surfaces with invalid alphaType can't be created, removed
unnecessary check for src's alphaType in asyncReadPixels tests.
Bug: b/238756873
Change-Id: Iddece4df686ccc38bca7e6303ab55025a0fd8f16
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/599198
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed
