Add GrMtlTexture classes

Adds support for basic Texture creation.

Bug: skia:
Change-Id: I9a3f15bef1c88054c19e952e231cad94ad69f296
Reviewed-on: https://skia-review.googlesource.com/30781
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
7 files changed