This enables the copyTexImage2D and copyTexSubImage2D to handle
multisampled source through through the blit path. We attempt to use a
cached texture as an intermediate to resolve to, or create a new texture
if there's a mismatch. The resolved texture is sent through
blit11::copyTexture as normal.

BUG:angleproject:2316
Change-Id: Ie3490b45b1a368300ee13fe5a0b35dc3920364ff
Reviewed-on: https://chromium-review.googlesource.com/911889Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>