capture button up events even if they go outside canvas

[andrewrk]

1. focus canvas
2. press Ctrl+tab to go to a new tab
3. click back on the game tab
4. chem thinks ctrl is still held down but it's not

[andrewrk]

not going to be solvable by putting a handler on window. I think the best that we can do is on mousemove check for event.ctrlKey etc and fire the up events if necessary.