vp9: Screen-content after slide-change: increase refresh rate
For screen-content real-time CBR mode: on a detected slide change
that is encoded at max Q (to prevent excessive overshoot), increase
the perc_refresh in the cyclic refresh following the slide change.
Use counter to increase refresh up to some #frames from slide change.
This is attempt to increase quality ramp-up after slide change without
causing too much excess overshoot.