Add missing WebGL extensions

[avaer]

We advertise the list of supported extensions here.

However not all of them are returned from gl.getExtension().

This resolves to returning the appropriate extension object with constants (or even an empty object). We should go through this list and make sure all of the extensions are returning their extension objects, and make sure that all of the requisite functionality like gl. constants is hooked in.

Side note

For some constants in these extensions we need to do an internal format translation from WebGL(GLES) to GL. That is performed by this constants table.