Add support for TS/RA2 split-shadows.

This adds 3 new fields to sequences:

* **ShadowStart** sets the start index of the shadow frames for the sequence (defaults to -1, i.e no shadow).
* **ZOffset** sets a relative z-offset for the sequence (to be used for overlays; defaults to 0).
* **ShadowZOffset** sets a relative z-offset for the shadow sequence (defaults to -5).