Add support for using texel buffers in Vulkan backend.

Bug: skia:
Change-Id: I5cc7b18263dbe28d5d8d89301111ef240109704a
Reviewed-on: https://skia-review.googlesource.com/15770
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
7 files changed