Make one-way recolors check for specific files

So, 2.3 added recoloring one-way tiles to no longer make them be always
yellow. However, custom levels that retexture the one-way tiles might
not want them to be recolored. So, if there are ANY custom assets
mounted, then the one-ways will not be recolored. However, if the XML
has a <onewaycol_override>1</onewaycol_override> tag, then the one-way
will be recolored again anyways.

When I added one-way recoloring, I didn't intend for any custom asset to
disable the recoloring; I only did it because I couldn't find a way to
check if a specific file was customized by the custom level or not.

However, I have figured out how to do so, and so now tiles.png one-way
recolors will only be disabled if there's a custom tiles.png, and
tiles2.png one-way recolors will only be disabled if there's a custom
tiles2.png.

In order to make sure we're not calling PhysFS functions on every single
deltaframe, I've added caching variables, tiles1_mounted and
tiles2_mounted, to Graphics; these get assigned every time
reloadresources() is called.

desktop_version/src/FileSystemUtils.h

@@ -17,7 +17,6 @@ bool FILESYSTEM_isMounted(const char* filename);
 
 void FILESYSTEM_mount(const char *fname);
 void FILESYSTEM_loadZip(const char* filename);
-extern bool FILESYSTEM_assetsmounted;
 void FILESYSTEM_mountassets(const char *path);
 void FILESYSTEM_unmountassets(void);
 bool FILESYSTEM_isAssetMounted(const char* filename);