vo_opengl: fix a cast

Basically, the OpenGL API is crap (it takes an offset as pointer).

video/out/gl_utils.c

@@ -222,7 +222,7 @@ static void gl_vao_enable_attribs(struct gl_vao *vao)
 
         gl->EnableVertexAttribArray(n);
         gl->VertexAttribPointer(n, e->num_elems, e->type, e->normalized,
-                                vao->stride, (void*)e->offset);
+                                vao->stride, (void *)(intptr_t)e->offset);
     }
 }