Rework supertx segment handling and adaptive quantization.

Segment level quantizer settings for supertx coded blocks are now
selected based on the minimum of all segment IDs within a supertx
coded block.

This also fixes the 3 adaptive quantization modes with supertx.

Change-Id: Ib5db099539d4f82f240e1d745d6e5264f8b34cde
4 files changed