enable selecting&transmitting to for intra mode entropy

This commit added a 3 bit index to the bitstream, the index is used to
look into the intra mode coding entropy context table. The commit uses
the mode stats to calculate the cost of transmitting modes using 8
possible entropy distributions, and selects the distribution that
provides the lowest cost to do the actual mode coding.

Initial test show this provides additional .2%~.3% gain over quantizer
adaptive intra mode coding. So the adaptive intra mode coding provides
a total of .5%(psnr) to .6% gain(ssim) combined for all-key-encoding

To build and test, configure with
--enable-experimental --enable-qimode

Change-Id: I7c41cd8bfb352bc1fe7c5da1848a58faea5ed74a