Improve quantizer handling

Improve quantizer handling
The default value for i_qpplus1 in x264_picture_t is now X264_QP_AUTO.  This is currently 0, but may change in the future.
qpfiles no longer use -1 to indicate "auto"; QP is just omitted.  The old method should still work though.

CRF values now make sense in high bit depth mode.
--qp should be used for lossless mode, not --crf.
--crf 0 will still work as expected in 8-bit mode, but won't be lossless with higher bit depths.
Add bit depth to statsfiles.

These changes are required to make the QP interface sensible in combination with high bit depth.