vp8: Add extra conditon for overshoot-drop

For drop due to large overshoot feature (in 1 pass CBR):
add additional condition that current prediction error
is larger than that of last encoded frame. This make the
drop due to sudden overshoot more robust, and improves
rate convergence for steady hard content.

Change-Id: If20027d26b4dcd290e4f788ae8e2760d95b536a5
3 files changed