On tiling GPUs, render pass loadOp and stencilLoadOp can be used to very
cheaply clear an image as it is being render to.  This change uses this
feature to clear render targets when possible.

Bug: angleproject:2361
Change-Id: Ic4bdc908873f4802760d549f4893f84a47beac0f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1500576
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>