Check for WEBGL_provoking_vertex extension

[mrxz]

Depending on the exact system configuration, the preferred provoking vertex convention might differ from the WebGL/OpenGL default. This can be communicated by the browser by exposing the WEBGL_provoking_vertex extension. In other words, the presence of this extension itself indicates it's worth changing the provoking vertex. This PR checks for the extension and sets FIRST_VERTEX_CONVENTION_WEBGL if present.

Going by the Web3D survey data (https://web3dsurvey.com/webgl2/extensions/WEBGL_provoking_vertex) this is relevant for the vast majority of Windows and Mac and iOS devices. Testing on a Windows laptop did show a really nice jump in performance.

This only applies when using flat interpolation in shaders, which Spark does. Since these values should be the same for each vertex, changing the provoking vertex makes no semantic difference. That said, it is a global configuration, meaning other uses might be impacted if they rely on flat interpolation picking the last vertex.