If vertexPipelineStoresAndAtomics is not available, the vertex stage
limit for images is set to 0, so there won't be any images in that
stage.  Similarly for fragmentStoresAndAtomics.  The extension can still
be exposed for use with compute shaders.

Bug: angleproject:3578
Change-Id: I86133415bc14c219067827758e727ce7ab079973
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2536252Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>