Only re-color one-ways if assets are not mounted

Some levels (like Unshackled) have decided to manually re-color the
one-way tiles on their own, and us overriding their re-color is not
something they would want. This does mean custom levels with custom
assets don't get to take advantage of the re-color, but it's the exact
same behavior as before, so it shouldn't really matter that much.

I would've liked to specifically detect if a custom tiles.png or
tiles2.png was in play, rather than simply disabling it if any asset was
mounted, but it seems that detecting if a specific file was mounted from
a specific zip isn't really PHYSFS's strong suit.

desktop_version/src/FileSystemUtils.h

@@ -15,6 +15,7 @@ char *FILESYSTEM_getUserLevelDirectory();
 
 bool FILESYSTEM_directoryExists(const char *fname);
 void FILESYSTEM_mount(const char *fname);
+extern bool FILESYSTEM_assetsmounted;
 void FILESYSTEM_mountassets(const char *path);
 void FILESYSTEM_unmountassets();