T8x8 experiment merge.

For ease of testing and merging experiments I have
removed in line code in encode_frame() that assigns
MBs to be t8x8 or t4x4 coded segments and have
moved the decision point and segment setup to
the init_seg_features0 test function.

Keeping everything in one place helps make sure
for now that experiments using segmentation are
not fighting each other.

Also made sure mode selection code can't choose 4x4
modes if t8x8 is selected.

Patch2: In init_seg_features() add checks
for SEG_LVL_TRANSFORM active.

Change-Id: Ia1767edd99b78510011d4251539f9bc325842e3a