Vulkan: Add Features class. This class will control optional features in the Vulkan back-end. It allows toggling the feature support from a centralized place. This can be useful for performance or correctness testing. Add a placeholder feature for line segment raster. We can also use a feature for enabling backbuffer flipping. Bug: angleproject:2598 Bug: angleproject:2673 Change-Id: I8ddec2dba2181d5b014267be68aee9d2cb015ccf Reviewed-on: https://chromium-review.googlesource.com/1120149 Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>