[fix] Add CanvasContext to wbuilder (#5287)
[fix] Add CanvasContext to wbuilder (#5287)

The widget builder was broken by <https://github.com/koreader/koreader/pull/4580>.
mihailim/koreader,Frenzie/koreader,Frenzie/koreader,NiLuJe/koreader,Hzj-jie/koreader,NiLuJe/koreader,koreader/koreader,poire-z/koreader,Markismus/koreader,mwoz123/koreader,pazos/koreader,poire-z/koreader,koreader/koreader