Clickable area according to bitmap

[nbrr]

Example draft for #22: it displays the cone pine image and moves it randomly when one tapes on the non-transparent area. Swipe exits. I did it quick & dirty as an example to see how it works out, it needs be abstracted.

Also relevant to #21: indeed we can just use the move semantics; I also had in my mind that maybe one could want to manipulate the universe outside the callback but the way it is now is not limiting and I can't come up with a scenario interesting enough for now.

Also relevant to #18: introduced a couple of fixes to panics introduced by the switch to u32 pixels.