This de-duplicates functionality from TextureStorage11_EGLImage and
TextureStorage11_2DMultisample. Neither of those supports changing the
associated images or image access in shaders.

Multisampled textures also cannot be compressed, so the size doesn't
need to be adjusted according to block size.

BUG=angleproject:2275
TEST=angle_end2end_tests

Change-Id: I45bc573584065bf4f528f883435fadfbd7281077
Reviewed-on: https://chromium-review.googlesource.com/1216002Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>