Don't include any GL header from the public GL headers

This splits the GL header inclusion from cogl-defines.h into a
separate headear called cogl-gl-header.h which we will only include
internally. That way we don't leak GL declarations out of our public
headers. The texture functions that were using GLenum and GLuint in
the public header have now changed to just use unsigned int. Note
however that if an EGL winsys is enabled then it will still publicly
include an EGL header. This is a bit more awkward to fix because we
have public API which returns an EGLDisplay and we can't determine
what type that is.

There is also a conformance test which just verifies that no GL header
has been included while compiling. The test isn't added to
test-conform-main because it doesn't actually test anything at
runtime.

Reviewed-by: Robert Bragg <robert@linux.intel.com>