Vulkan: Expose OES_shader_image_atomic without VS/FS atomics 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/+/2536252 Reviewed-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>