Fix return value of PHYSFS_fileLength() being stored in a smaller size

The function returns a PHYSFS_sint64, but we forcefully shove it into a
PHYSFS_uint32. This means we throw away all the negative numbers, which
is bad because the function returns -1 if the size of the file can't be
determined; plus, we also throw away 32 bits of information, reducing
our range of supported file sizes from 9 exabytes to 4 gigabytes.

File size support is only as good as the weakeast link, and it looks
like one of the consumers of FILESYSTEM_loadFileToMemory(),
SDL_RWFromConstMem(), only takes in a signed 32-bit integer of size;
however, I would still like to do at least the bare minimum to support
as many file sizes as we can, and changing types around is one of those
bare minimums.

desktop_version/src/FileSystemUtils.cpp

@@ -360,7 +360,7 @@ void FILESYSTEM_loadFileToMemory(
 	{
 		return;
 	}
-	PHYSFS_uint32 length = PHYSFS_fileLength(handle);
+	PHYSFS_sint64 length = PHYSFS_fileLength(handle);
 	if (len != NULL)
 	{
 		*len = length;