Fixes GLExtensions for GL 2.1 or GLES 2 devices.

This wasn't too much of a concern since we normally don't care about this feature set, but it is nice when testing on new devices and they don't
support the higher feature sets but want to run under software renderer.

The Mesa softpipe and PowerVR 5xx drivers don't support higher GL versions, but they shouldn't exit out just because they couldn't get a GL3 function
pointer that isn't even going to be used at that point.
Merge pull request #1566 from Sonicadvance1/fix_gl21

Fixes GLExtensions for GL 2.1 or GLES 2 devices.