paint example slightly misleading with respect to mouse_cursor_args

[radix]

event.mouse_cursor_args does not return coordinates when the event has a MouseButton event -- an odd behavior, for sure, but the paint example specifically calls it anyway when it receives a mouse click. It also calls it elsewhere, but the call in the handler for MouseButton was enough to confuse me for a couple of hours as I tried to get mouse input working.

I'll submit a PR to remove that call.

[radix]

I'll close this since the PR was merged, thanks :)