Add a speed feature to skip transform type selection

Add a speed feature to skip transform type selection

Setting FIXED_TX_TYPE as 1 makes the encoder skip tx_type search,
about twice as fast.

This speed feature is off by defualt; we can turn it on when we
want to quickly test new ideas.

Change-Id: Ieab5807d17fcd54fce3e8ae2f59a18b42eb79408