Support vertex-only render pipeline on D3D12, Vulkan, Metal, OpenGL
and OpenGL ES backends. Related validation tests and end to end tests
are also implemented.
Bug: dawn:136
Change-Id: If266fd441c1d39ccd940ea26b74b405f8abb351a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/63080
Commit-Queue: Zhaoming Jiang <zhaoming.jiang@intel.com>
Reviewed-by: Austin Eng <enga@chromium.org>