cursor: Fix touch point surface attachment

An ugly patch that fixes plenty of issues with touch events we had (drag'n'drop, touch part of #52, gtk's selection and copy'n'paste interface etc.), but does it in a way that's not exactly elegant.

Touch points should remain attached to surface they originate from regardless of the touch point position on screen.

The main problem with that is that we need to translate layout coordinates into local coordinates of a given surface... which isn't easy. I'll keep looking for different ways of achieving that, without iterating through all views and roots_layer_surfaces (which doesn't even handle xdg_popups), but couldn't think of any so far.

(and it's completely untested on XWayland, which may add its own set on challenges)