Rearrange MpegEncContext to simplify access from asm

This moves the fields needed by asm near the top, before any
structs or other members which complicate the offset calculation.
Modifying other structs will no longer require updating the offsets,
and the asm code is slightly simpler due to the smaller offsets.

Signed-off-by: Mans Rullgard <mans@mansr.com>