Use vkCmdClearColorImage to clear backend textures

Also don't use GrPixelConfig to create the VkImage.


Bug: skia:7959
Bug: skia:6718

Change-Id: Ia13c5ed2fbe0542c060b725694eff9d566c491f0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226078
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed