EXT_provoking_vertex for WebGL2

I don't think there's ever been a provoking vertex extension for OpenGL ES, but being able to control the provoking vertex convention has a unique applicability to WebGL2 as a way of improving the efficiency of flat-shaded rendering on certain platforms.

Flat-shaded rendering with the "provoking vertext last" convention must be supported by WebGL2 implementations, but some implementations that layer it on top of graphics APIs that only support the "provoking vertex first" convention have to emulate this behavior, so there's no way for an application to tell if it will be efficient. If running on a WebGL2 implementation that supported an optional extension similar to EXT_provoking_vertex, an application could be confident that flat-shaded rendering with the "provoking vertex first" convention would be efficient. If not, it could fall back to using index buffers built for the "provoking vertex last" convention, which would likely be efficient if said extension were widely implemented on platforms where that convention had to be emulated.