When the user requests an IOSurface Pbuffer with an RGB format, emulate the
missing alpha channel by clearing it to 1.0 and masking reads and writes in
shaders.

BUG=angleproject:3766

Change-Id: I58c992bf641d9ece0f923603f32640615150e4f3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1737437
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>