implement eglGetProcAddress

According to spec:

[eglGetProcAddress may be queried for all GL and EGL extension functions.](http://www.khronos.org/registry/egl/sdk/docs/man/xhtml/eglGetProcAddress.html)

Implemented for GL functions. EGL functions I believe are only needed for extensions and the normal calls are always exported, but I may be wrong about this.