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.cpp

@@ -149,6 +149,7 @@ void Graphics::init(void)
 #ifndef NO_CUSTOM_LEVELS
     tiles1_mounted = false;
     tiles2_mounted = false;
+    minimap_mounted = false;
 #endif
 }
 
@@ -3402,6 +3403,7 @@ void Graphics::reloadresources(void)
 #ifndef NO_CUSTOM_LEVELS
 	tiles1_mounted = FILESYSTEM_isAssetMounted("graphics/tiles.png");
 	tiles2_mounted = FILESYSTEM_isAssetMounted("graphics/tiles2.png");
+	minimap_mounted = FILESYSTEM_isAssetMounted("graphics/minimap.png");
 #endif
 }