video_output: add 1:0 special aspect ratio to fill the whole display

Draft on top of !6048 (merged). Only the last 2 commits are new.

As discussed in !5991 (closed), it makes more sense to stretch the video to fill the display with an aspect ratio.

The special value is used internally to adjust the vd->source SAR so the placement ends up filling the display area. So we need to make sure the display modules use vd->source (with odd SAR) to place their video in the display.