Vulkan: Add a feature related to shader subsets On vendors that support VK_EXT_graphics_pipeline_library the shaders subset of the pipeline can either be combined into a single pipeline or can be decoupled into separate pipelines. Upcoming optimizations require the ability to distinguish between the two options. Add a feature declaring support for only the combined variant. Bug: angleproject:8601 Change-Id: I44fb828d972ca6ebba5549819ce8e41645683e14 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5484673 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>