JSI HUD fixes.

1 - Flipped the HUD ladder pixel matrix so that the prograde icon was not inverted.
2 - Changed the iconMaterial to use the unlit shader instead of Alpha/Unlit Transparent.  Fixes missing prograde icons on transparent HUDs.
3 - Added showHeadingBarPrograde to allow suppressing the prograde icon on the heading strip (helldiver feature request).
Add NEXTAPSIS accessors.

They provide altitude and time to the next apsis (peri- or apo-).  Also a variable to indicate which apsis is being returned.