Add FILESYSTEM_isAssetMounted()

This function will check if a specific file is a mounted per-level
custom asset, instead of being a variable that's true if ANY file is a
mounted asset.

desktop_version/src/FileSystemUtils.h

@@ -20,6 +20,7 @@ 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);
 
 void FILESYSTEM_loadFileToMemory(const char *name, unsigned char **mem,
                                  size_t *len, bool addnull = false);