- call `on-size' on setting the canvas size, like Win32 and Cocoa,
which mainly shows/hides auto-shown scrollbars more on time
(e.g., DrRacket doesn't start with horizontal scrollbars that
immediately disappear)
- switch mouse tracking to "hint" mode, instead of catching every
single mouse position
- when queueing `on-size' methos in response to client-size changes,
cancel and queued but undelivered calls; in the case of editor
canvases, this doesn't really have an effect, due to the other
`on-size' call
Closes PR 11369, hopefully
