OpenGL2: Fix flares behind mirror being visible

When r_ext_framebuffer_multisample > 0 was used, flares behind the
mirror in q3tourney6 were incorrectly visible. This was because it
checks scene depth in the depth prepass which only drew opaque
surfaces. It also needs to contain depth for mirror/portal surfaces.

code/renderergl2/tr_backend.c

@@ -455,7 +455,7 @@ void RB_RenderDrawSurfList( drawSurf_t *drawSurfs, int numDrawSurfs ) {
 
 	for (i = 0, drawSurf = drawSurfs ; i < numDrawSurfs ; i++, drawSurf++) {
 		if ( drawSurf->sort == oldSort && drawSurf->cubemapIndex == oldCubemapIndex) {
-			if (backEnd.depthFill && shader && shader->sort != SS_OPAQUE)
+			if (backEnd.depthFill && shader && (shader->sort != SS_OPAQUE && shader->sort != SS_PORTAL))
 				continue;
 
 			// fast path, same as previous sort
@@ -484,7 +484,7 @@ void RB_RenderDrawSurfList( drawSurf_t *drawSurfs, int numDrawSurfs ) {
 			oldCubemapIndex = cubemapIndex;
 		}
 
-		if (backEnd.depthFill && shader && shader->sort != SS_OPAQUE)
+		if (backEnd.depthFill && shader && (shader->sort != SS_OPAQUE && shader->sort != SS_PORTAL))
 			continue;
 
 		//