Revert "GLES2: upgrade gl2ext.h to version 16994."

Revert "GLES2: upgrade gl2ext.h to version 16994."

This reverts commit 8818b88748bde6de5a4a98ad237c85151b44c774.

I get a lot of errors like this one:

In file included from ../../../src/mapi/glapi/glapi_priv.h:49:0,
                 from glapi_dispatch.c:40:
../../../include/GLES2/gl2ext.h:1074:28: error: redefinition of typedef ‘PFNGLRENDERBUFFERSTORAGEMULTISAMPLEEXTPROC’
../../../include/GL/glext.h:10237:25: note: previous declaration of ‘PFNGLRENDERBUFFERSTORAGEMULTISAMPLEEXTPROC’ was here

This with a clean build (with git clean -fdX).

I don't get the errors on my other machine. I didn't investigate why,
a wild guess is that this depends on the version of gcc.