When the angleFormat we have has no alpha channel, but the actual
texture underneath has one, we shouldn't be drawing over the alpha
channel, so we apply a mask on it when we're in this situation.

Bug: angleproject:2597
Change-Id: Ia7110709e6ee32bb61988d08f5049e4e80e7e24e
Reviewed-on: https://chromium-review.googlesource.com/1106759
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>