On the encoder side (encoder.c):
* seq_force_integer_mv is initialised to 2 (and never changed)
* if allow_screen_content_tools == 0, cur_frame_force_integer_mv is set to 0
* if allow_screen_content_tools == 1, then
  - if seq_force_integer_mv == 2, cur_frame_force_integer_mv = is_integer_mv(..)
  - otherwise cur_frame_force_integer_mv = seq_force_integer_mv

This behaviour is now reflected in the functions that read/write the
uncompressed header, with suitable asserts on the encoder side to ensure that
this is in fact what is occurring.

BUG=aomedia:1338

Change-Id: Ic508a9b2188002c6ea26501c142ed58cce98be6a