Add Texture usage to GrAttachment.

This is not used anywhere yet, just starting to get the framework all
set up.

Bug: skia:10727
Change-Id: Ie321bb850e4c0eecd5e557b8a7828ee941ba7d2c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/378377
Reviewed-by: Brian Salomon <bsalomon@google.com>
9 files changed