Refine VkYcbcrSamplerHelper to only hold backend memory for YCbCr textures

This arrangement allows the backend texture to outlive the YCbCr SkImage.

Change-Id: I34939d05bf1091c8efcacb687dc1900729d4cbe5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296478
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed