Always use GL_Cull to change cull state

Manually changing cull state can cause later GL_Cull calls to not change
the cull state.

code/renderergl2/tr_backend.c

@@ -818,7 +818,7 @@ void	RB_SetGL2D (void) {
 			  GLS_SRCBLEND_SRC_ALPHA |
 			  GLS_DSTBLEND_ONE_MINUS_SRC_ALPHA );
 
-	qglDisable( GL_CULL_FACE );
+	GL_Cull( CT_TWO_SIDED );
 	qglDisable( GL_CLIP_PLANE0 );
 
 	// set time for 2D shaders