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/1120149Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>