This commit makes ext-tx and rect-tx experiments supported in the
cb4x4 mode. It resolves an enc/dec mismatch issue when all the
transform experiments are enabled.

The coding gains are
        ext-tx + rect-tx   cb4x4    vartx     total
lowres      4.0%           2.3%      0.5%     6.9%

The encoding speed is about the same when cb4x4 and vartx are
further enabled.

BUG=aomedia:139

Change-Id: I3fdabc6d5de23ceb78ac0751a9bf7332ebc0a3ac