This is primarily in preparation for compressed texture copy, but has
the following side effect:

- When transfer is possible, it's faster than draw
- When texture format does not support draw (but transfer is possible),
  it will avoid copying through CPU.

Bug: angleproject:2670
Change-Id: I49e1b51e6ccec875db3f971106687c7d48c4916f
Reviewed-on: https://chromium-review.googlesource.com/c/1470595
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>