vk: Hold refs to sampled textures as opposed to raw ptrs

Bug: skia:8776
Change-Id: Ie2956d0d6df627b7689bfc2765173e1042563d28
Reviewed-on: https://skia-review.googlesource.com/c/194006
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed