Properly document how videomixing works

Submitted by Kim Lam

Link to original bug (#712714)

Description

I am trying to display one video frame ontop (spatially) of a second video frame. The behavior of the smart-mixer is interfering with this, because it is resizing the video frame whenever a timeline commit occurs. It would be nice to be able to fix the size of the output canvas in which video frames are drawn on. This would be useful for compositing from multiple video sources.

Version: 1.2.0