Add row-mt=1 to vp9 encoding preset.
Add row-mt=1 to vp9 encoding preset.

This significantly improves speed with very minor quality loss:
https://github.com/mltframework/shotcut/pull/491
https://groups.google.com/a/webmproject.org/forum/#!topic/webm-discuss/
G_fuix013KM

j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt