Some initial cleanup to aid testing and debug.

Pull code to choose temporal or spatial encoding
out of encodeframe.c into a dedicated function
in segmentation.c.

For now disable broken temporal mode.

Move the coding of "temporal_update" flag and
only transmit if segment map update is indicated.

Rename the functions read_mb_features() and
write_mb_features() to read_mb_segid() and
read_mb_segid() as they only read and write
the macroblock segment id not any of the
features.

Change-Id: Ib75118520b1144c24d35fdfc6ce46106803cabcf