Resolved experimental branch build problem when seg_features not configured.

Change-Id: Ia0f9b460a26dc3eac9844ee595a7b196e9faf6a5

Only encode sign bit for feature data that can have a sign.

Tweaks to the test segmentation rules so that it now actually gives
a net benefit on the derf set of about 0.4% though much higher
on some clips at the low end.

Change-Id: I8e61f1aebf41c9037db7e67e2f8975aa18a0c986

This quite large check in includes the following:

Merge in some code from Ronald (mbgraph.c) that scans a Gf/arf group.
This is used as a basis for a simple segmentation for the normal frames
in a gf/arf group. This code also uses satd functions from Yaowu.

Adds functionality for coding the latest possible position of an EOB for
blocks in the segment. (Currently 0-15 only, hence just for 4x4 dct).
Where the EOB position is 0 this acts like "skip" and the normal coding
of skip at the per mb level is disabled.

Added functions (seg_common.c) for setting and reading segment feature
elements. These may want to be optimized away at some point but while the
mecahnism is in a state of flux they provide a single location for making
changes and keep things a bit cleaner.

This is still proof of concept code. Currently the tested feature set:-

Quantizer,
Loop Filter level,
Reference frame,
Prediction Mode,
EOB end stop.

TBD:-

Add functions for setting and reading the feature...