Fix a bug in ml_prune_rect_partition()
Fix a bug in ml_prune_rect_partition()

The quantization step size should be scaled properly for high bit depth
settings.

This only affects speed 0.
Encoder speed change is almost neutral.
There is a small coding gain of 0.09%.

Change-Id: I96b2bae03a53ce8ccd6428e3a050cfe18e06a024

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx