With this change I also removed GrPixelConfig as param to supportedReadPixelsColorType().
This meant some updates had to be made to Vulkan and Metal to make sure they return the
right GrColorType.
Bug: skia:6718
Change-Id: I71b6360489cf499692c7b777e5915090fad05c56
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228349
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>