Updated bc7enc_rdo with improved smooth block handling

The command line tool now detects extremely smooth blocks and encodes them with a significantly higher MSE scale factor. It computes a per-block mask image, filters it, then supplies an array of per-block MSE scale factors to the ERT. -zu disables this. 

The end result is much less significant artifacts on regions containing very smooth blocks (think gradients). This does hurt rate-distortion performance.

(The second image was resampled to 1/4th res for blogger.)