This works by storing the deferred clears in the ImageHelper's staging
buffers. We apply the deferred clears onto the RenderPass right before
we begin to draw. Storing the clears in the ImageHelper solves problems
where we clear GL Textures in a Framebuffer and then unbind the
Textures and sample from them. Or do other commands like CopyTexImage.

Note that because the staging buffer clears only handle full-image
clears we need to immediately apply some scissored clears where before
we would use the RP. This should be a pretty rare occurrence and it is
possible to optimize that in the future.

Reduces the RenderPass count in the Manhattan "frame 10" trace from max
22 to max 20. May improve perf slightly on Android or may have effects
too small to measure. Should not regress performance.

Bug: angleproject:4517
Change-Id: I02150d531022afb903f1058f070937ec6337bd88
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2142711Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>