OpenSubdiv: Attempt to solve crash on certain GPUs/drivers

intern/opensubdiv/gpu_shader_opensubd_display.glsl

@@ -52,6 +52,10 @@ void main()
 {
 	outpt.v.position = modelViewMatrix * position;
 	outpt.v.normal = normalize(normalMatrix * normal);
+	/* Some compilers expects gl_Position to be written.
+	 * It's not needed once we explicitly switch to GLSL 1.40 or above.
+	 */
+	gl_Position = outpt.v.position;
 }
 
 #endif  /* VERTEX_SHADER */