Segment Feature Data Access

No change to functionality or output.

Updates to the segment feature data structure now all done
through functions such as set_segdata() and get_segdata()
in seg_common.c.

The reason for this is to make changing the structures (if needed)
and debug easier.

In addition it provides a single location for subsequent addition
of range and validity checks. For  example valid combination of
mode and reference frame.

Change-Id: I2e866505562db4e4cb6f17a472b25b4465f01add