Make mouse-actions near instantaneous

[GlenDC]

Users expect brush actions, as controlled by mouse actions, to react instantly in any situation. Think of any painting program and you'll notice that when you're dragging and painting that you'll exactly hit each spot as you would expect.

Right now this is not the case for trixel. Talking with the Elm community on issues we face, like this one, is probably a good idea, as they might have a better idea why this is happening at all.

[GlenDC]

Right now we have a lot of incoming signals merged via Signal.MergeMany, it might be a good idea to move the postActions (containing all the mouse and keyboard user-input) to the most-left position, so that it always wins. Although I'm fairly sure that this won't affect the speed in a way we would want.