Factor out "between" calculation to macro and func

FILESYSTEM_mountAssets() has a big comment describing the magic numbers
needed to grab FILENAME from a string that looks like
"levels/FILENAME.vvvvvv".

Instead of doing that (and having to write a comment every time the
similar happens), I've written a macro (and helper function) instead
that does the same thing, but clearly conveys the intent.

I mean, just look at the diff. Using VVV_between() is much better than
having to read that comment, and the corresponding SDL_strlcpy().

desktop_version/src/FileSystemUtils.cpp

@@ -325,25 +325,12 @@ void FILESYSTEM_loadZip(const char* filename)
 
 void FILESYSTEM_mountAssets(const char* path)
 {
-	const size_t path_size = SDL_strlen(path);
 	char filename[MAX_PATH];
 	char zip_data[MAX_PATH];
 	const char* zip_normal;
 	char dir[MAX_PATH];
 
-	/* path is going to look like "levels/LEVELNAME.vvvvvv".
-	 * We want LEVELNAME, which entails starting from index 7
-	 * (which is how long "levels/" is)
-	 * and then grabbing path_size-14 characters
-	 * (14 chars because "levels/" and ".vvvvvv" are both 7 chars).
-	 * We also add 1 when calculating the amount of bytes to grab
-	 * to account for the null terminator.
-	 */
-	SDL_strlcpy(
-		filename,
-		&path[7],
-		VVV_min((path_size - 14) + 1, sizeof(filename))
-	);
+	VVV_between(path, "levels/", filename, ".vvvvvv");
 
 	SDL_snprintf(
 		zip_data,