Add finalmode check to drawfinalmap call

This fixes the finalstretch tile shifting persisting if you return to
the main dimension and final_colormode isn't reset properly.

It's possible to do so in the main game by using a teleporter in
finalmode while having the Intermission 1 or 2 companion active.

For custom levels, level makers can make a setup that automatically
turns on finalstretch, goes to finalmode, and then returns to the main
dimension. The only thing being... as a level maker myself, this tile
shifting REALLY doesn't seem useful (and no one has ever used it because
the setup to do so hadn't really been found or documented until this
year). For one, the exact shift is randomized every time (there's an
fRandom() call to cycle the colors). For two, it goes away after the
player saves and reloads the level. And for three, it doesn't animate
like it does in finalmode (this is the biggest reason IMO).

Nevertheless, I've decided to keep support for this in custom levels, in
case someone in the future does want to use it and is okay with the
limitations.

desktop_version/src/Render.cpp

@@ -1560,7 +1560,7 @@ void gamerender(void)
             {
                 ClearSurface(graphics.backBuffer);
             }
-            if (map.final_colormode)
+            if ((map.finalmode || map.custommode) && map.final_colormode)
             {
                 graphics.drawfinalmap();
             }