Rename GrStencilAttachment class to generic GrAttachment

Additional this adds a UsageFlags member to the new GrAttachment
class.

Bug: skia:10727
Change-Id: Ifc0bfffd959f5fbc46bfcdf269e1b2a933929753
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/323107
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
65 files changed