If we update the texture data between two dispatch calls, ReadPixels can't
get right result after the second dispatch call. The failure reason is
that after the first dispatch, ReadPixels will sync framebufer state which
will update color render target. Finally, TextureD3D::ensureRenderTarget
is reached. However, we are in compute pipeline.
mTexStorage->isRenderTarget() will be false. That results the current
texture will create a new render target storage. But the UAV is still
bound with the previous texture storage. If there is no texture dirty bit
between these two dispatch calls, applyTexturesForCompute won't be called.
After the second dispatch, readPixels will read data from the new texture
storage which is not updated.

Bug: angleproject:3015

Change-Id: Ib2494ab8bf6e12faefc0a7370719d383526c36ba
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1390710Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>