Move GrGLAttachment to a factory model similar to other backends.

Bug: skia:12215
Change-Id: Ibd8d46c8e36bbde6a15f214dfc376622509dc199
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/428976
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
7 files changed