Use lossless for Q0

The commit changes the coding mode to lossless whenever the lowest
quantizer is choosen.

As expected, test results showed no difference for cif and std-hd
set where Q0 is rarely used. For yt and yt-hd set, Q0 is used for
a number of clips, where this commit helped a lot in the high end.

Average over all clips in the sets:
yt: 2.391% 1.017% 1.066%
hd: 1.937%  .764%  .787%

Change-Id: I9fa9df8646fd70cb09ffe9e4202b86b67da16765