gtk/gl: Only unbind buffers/vertex attrib arrays if we can't directly bind the vertex...

gtk/gl: Only unbind buffers/vertex attrib arrays if we can't directly bind the vertex array to 0

Binding the vertex array to 0 will unbind everything else already.

In the previous order older versions of the Intel GL driver caused
errors to be printed for every single call when disabling the vertex
attrib arrays after binding the vertex array to 0.