Add kRGB_888X_GrPixelConfig

This is needed to support importing of RGBX AHB into skia's vulkan backend.
With this CL we only enable the new pixel config to be textureable.

Bug: skia:
Change-Id: Iba9180c14f3ef633ae846091cf453d68f82ce544
Reviewed-on: https://skia-review.googlesource.com/c/192035
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
17 files changed