Enables stereo rendering with OpenGL ES 3.1 + AEP.

If the host device supports GLES 3.1 and AEP we can have stereo rendering.
Just need to make sure to grab the correct function pointer that GL_EXT_geometry_shader provides, and enable AEP in the shaders.

We can't just check if AEP is in the extension list for support because Qualcomm has failed once more.
With the Nexus 6 it reports support for AEP but doesn't support OpenGL ES 3.1, which is an impossible combination.
From reports on their forum it seems that attempting to use any AEP things results in nothing happening, seems like a stub implementation.
Merge pull request #1664 from Sonicadvance1/GLES3_stereo

Enables stereo rendering with OpenGL ES 3.1 + AEP.