Draw minimap.png if it is mounted

This is a simple change - we draw minimap.png, instead of the generated
custom map, if it is a per-level mounted custom asset.

Custom levels have already been able to utilize minimap.png, but it was
limited - they could do gamemode(teleporter) in a script, and that would
show their customized minimap.png, but it's not like the player could
look at it during gameplay.

I would have done this earlier if I had figured out how to check if a
specific asset was mounted or not.

desktop_version/src/Graphics.h

@@ -220,6 +220,7 @@ public:
 #ifndef NO_CUSTOM_LEVELS
 	bool tiles1_mounted;
 	bool tiles2_mounted;
+	bool minimap_mounted;
 #endif