Native R8 support for sequences

This cleans up and enables the R8Reader for ingame sequences (but not cursors).
See 56e7925 for example vehicle, infantry, and turret sequences.

Sprites are centered on the reference point defined in the frame header (which isn't related to the frame size at all). For vehicles this is the center of the unit; infantry is at their feet, and buildings is something weird that may be well outside the sequence artwork. The Offset sequence key can be used to reposition the artwork to its desired position.