Fix shared OpenGL/GLES functions not being shared
From the important commit:
Cogl shares some GL functions between the GLES and the big GL drivers. Namely, it shares _cogl_driver_gl_context_init and _cogl_driver_gl_context_deinit between these two drivers. The plot twist is: even though these functions are shared and their prototypes are in cogl-util-gl-private.h, they're actually implemented inside cogl-driver-gl.c, which is strictly only about the big GL driver. This is problematic when building Mutter on ARM v7, where we need to disable OpenGL, but keep GLES enabled. Fix this by moving the shared GL functions to a shared GL file.