render_map: Cannot render maps that contain groups with negative offsets

[heinrich5991]

Example map: Dirty_RaiNy.map (SHA256: 42181f7091f503fc36a552853fdeb76e69e9d839d09a06d08749d2691bd7279c).

[heinrich5991]

It seems harder than expected to do this, because the offsets support values lower than the width of one tile, or more precisely, they support steps of 1/32 tile width.