winex11: When a mask picture is active, the source offsets should be relative to mask_pict instead of src_pict.

For a Mac user in Australia, for example, the out-of-the-box configuration is
that the preferred language is generic English ("en").  Their formats locale
should be Australia ("en_AU").  Detect that case and don't override
LC_MESSAGES.

This enables it to be auto-updated by make_makefiles.