Adds support for OpenGL ES draw_elements_base_vertex.

This is the same extension that we all know and love but under a different name with some different requirements.
In regular OpenGL fashion, you can't just move a desktop OpenGL extension to OpenGL ES without ratifying a new extension, which is why this falls
under a EXT extension, which in turn causes it to have suffixes attached to their function names.

This is the first step in our way towards conquering all mobile GPUs that don't support desktop OpenGL, hopefully we also can add support for
buffer_storage to OpenGL ES as well so we can make full use of this extension.
Merge pull request #1583 from Sonicadvance1/mobile-draw-elements-base-vertex

Adds support for OpenGL ES draw_elements_base_vertex.