Instead of using generic terms like "idx" and "number", explicitly
 describe every variable involving frame indices in terms of
 whether it is counting input frames or output frames.
This removes an output-centric bias that may not be obvious in some
 places (group_len vs. group_src_len), and immediately makes
 several bugs in guess_frame_subtypes() obvious.

This patch introduces no functional changes.
It is just renaming.

PUBLIC API CHANGE:
-Packet::number gets renamed