Adding an new extension, which is not present in the Vulkan headers,
in order for ANGLE to be able to specify to SwiftShader which
provoking vertex convention to use, in order to properly emulate
OpenGL ES on top of Vulkan, since OpenGL ES uses the last vertex
convention and Vulkan uses the first vertex convention.

Bug: angleproject:3677, angleproject:3430
Change-Id: If4e6539d48bc282af8ed6f2a9db6b2fc9d0dbe2d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37248
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>