Fix roomname being continuously drawn on top of itself

This is only noticeable if you have a font with translucent pixels, like
I do. But it gets really noticeable really quickly with this over-30-FPS
patch because the render functions are continuously called every
delta-timestep. To prevent this, just fill the backbuffer with black
before rendering anything.

desktop_version/src/Render.cpp

@@ -1627,6 +1627,8 @@ void gamerender()
 
 void maprender()
 {
+    FillRect(graphics.backBuffer, 0x000000);
+
     //draw screen alliteration
     //Roomname:
     int temp = map.area(game.roomx, game.roomy);
@@ -2392,6 +2394,7 @@ void maprender()
 
 void teleporterrender()
 {
+    FillRect(graphics.backBuffer, 0x000000);
     int tempx;
     int tempy;
     //draw screen alliteration