This splits up cogl-ext-functions.h in to sets of prototypes that
can be included separately so that we can include just core
gles1 or gles2 functions without any extensions.
Since eglGetProcAddress can not be used to query core client APIs
and some implementations (notably on Android) can return a garbage
pointer instead of NULL this will allow us to explicitly check
when to use eglGetProcAddress and when to use dlsym().
Reviewed-by: Neil Roberts <neil@linux.intel.com>
