Fix gr_end_2d_matrix viewport for render-to-texture

`gr_set_proj_matrix` already branches on rendering_to_texture to choose
top-left (RTT) vs bottom-left (screen) viewport origin. `gr_end_2d_matrix`
should match, but it unconditionally used the bottom-left formula. Add
the same `rendering_to_texture` branch so the viewport is restored
correctly when rendering to a texture.

Author: laanwj

code/graphics/matrix.cpp

@@ -272,7 +272,11 @@ void gr_end_2d_matrix()
 	Assert( htl_2d_matrix_depth == 1 );
 
 	// reset viewport to what it was originally set to by the proj matrix
-	gr_set_viewport(gr_screen.offset_x, (gr_screen.max_h - gr_screen.offset_y - gr_screen.clip_height), gr_screen.clip_width, gr_screen.clip_height);
+	if (gr_screen.rendering_to_texture != -1) {
+		gr_set_viewport(gr_screen.offset_x, gr_screen.offset_y, gr_screen.clip_width, gr_screen.clip_height);
+	} else {
+		gr_set_viewport(gr_screen.offset_x, (gr_screen.max_h - gr_screen.offset_y - gr_screen.clip_height), gr_screen.clip_width, gr_screen.clip_height);
+	}
 
 	gr_projection_matrix = gr_last_projection_matrix;