Missing OpenGL ES 2 extension function aliases

[mosra]

Since the WEBGL_draw_buffers WebGL extension is based on EXT_draw_buffers ES2 extension and ANGLE_instanced_arrays WebGL extension is based on ANGLE_instanced_arrays ES2 extension, I thought it would make sense to expose also functions that are present in ES2 extension headers:

glDrawBuffersEXT()
glVertexAttribDivisorANGLE()
glDrawArraysInstancedANGLE()
glDrawElementsInstancedANGLE()

This should also partially fix #2510, except for additions to the headers (I'm using my own headers generated using flextGL so that doesn't bother me).

I looked around the code to discover what's the actual policy for extension function suffixes, but it seems that all suffixes of given function are always present without checking whether we are on legacy GL emulation, WebGL-friendly subset or else, so I just added these to existing aliases.

Note: if I understand correctly, the WEBGL_draw_buffers and ANGLE_instanced_arrays extensions shouldn't be exposed on WebGL 2 (because the above functions are then in core), thus aliases with these suffixes also shouldn't be present for WebGL 2, but maybe that's not a problem here.

Thanks!

[kripken]

@juj, what do you think?

[juj]

I'm ok with these being added. @mosra: Can you add your name to the file AUTHORS?(https://github.com/kripken/emscripten/blob/incoming/AUTHORS).

In desktop OpenGL, the convention is that implementations may continue to advertise an extension even after it is merged to core (and in the case the core functionality and the extension have functional differences, the differences will be preserved). I think we can just keep it simple and do the same here unconditionally.

If you have a small test available, that would be nice.

[mosra]

@juj: done.

Unfortunately I don't have any small and self-contained test, sorry.