vp9enc: performance optimization work

This mostly exposes a few new properties that allow the encoder to make better use of the available processing power.

On a 12-core machine:

gst-launch-1.0 -v videotestsrc ! video/x-raw, width=1920, height=1080 ! queue ! vp9enc threads=12 cpu-used=4 tile-rows=2 row-mt=true ! fakesink

uses around 450 % CPU, instead of around 180% with:

gst-launch-1.0 -v videotestsrc ! video/x-raw, width=1920, height=1080 ! queue ! vp9enc threads=12 cpu-used=4 ! fakesink