This logic was buried deep inside
 FrameInvariants::new_inter_frame(), but we will need it elsewhere
 to make guess_frame_subtypes() work.
Separate out the static configuration into InterConfig, and pass in
 the variable state as function parameters.
This patch makes new functions to
- Determine the index of an output frame in its reorder group
- Get the order hint for a given output frame
- Get the pyramid level of an output frame
- Get the reference frame slot where a decoded frame will be stored
- Get whether or not a given output frame will be shown
- Get whether or not a given output frame is a Show Existing Frame
Not yet converted:
- Logic to select the references used by an inter frame

This patch introduces no functional changes.