Implement robust initialization for image texture This change fixes a bug that the image texture initialization time is not correct. It happens when we use compute shader to write data to an uninitialized texture, then use readPixels to read the result which results that texture initialization falls behind image store and covers the real result. Bug: angleproject:2766 Change-Id: I4e986972096857afc975c40dfa4d559a2f31194c Reviewed-on: https://chromium-review.googlesource.com/1170569 Commit-Queue: Jiajia Qin <jiajia.qin@intel.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>