Fit the mplex directly instead of using region_fit, due to #3349390
Fit the mplex directly instead of using region_fit, due to #3349390

When we ever get to refactor Notion to make WRootWin not subclass
Screen/WMPlex (so we can allow screens to be resized again), this code
will no longer work, but yield an easy-to-fix compile-time error.

dkogan/notion,raboof/notion,p5n/notion,knixeur/notion,dkogan/notion,anoduck/notion,anoduck/notion,knixeur/notion,raboof/notion,neg-serg/notion,dkogan/notion,dkogan/notion.xfttest,p5n/notion,p5n/notion,neg-serg/notion,dkogan/notion.xfttest,knixeur/notion,dkogan/notion,knixeur/notion,p5n/notion,p5n/notion,dkogan/notion,raboof/notion,anoduck/notion,neg-serg/notion,knixeur/notion,raboof/notion,anoduck/notion,neg-serg/notion,dkogan/notion.xfttest,dkogan/notion.xfttest,anoduck/notion