Clean up TextureStorage11_2DMultisample 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/1216002 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>