gtk/gl: Only unbind buffers/vertex attrib arrays if we can't directly bind the vertex array to 0
(changed title to match commit)
That is, first disable the vertex arrays, then bind the buffers to 0 and only then bind the vertex array to 0 instead of the other way around.
In the previous order older versions of the Intel GL driver caused errors to be printed for every single call when disabling the vertex arrays.
CC @ystreet