Fix empty selection when user doesn't move their cursor

[rpigott]

Slurp has this strange behavior where you start slurp and the surface is immediately fullscreen, but if your cursor is already positioned in the selection area (when you provide your own boxes) it doesn't highlight them. Worse, if you click within a selection area without moving your cursor it doesn't output the selection area you clicked on!

This PR changes that so the selection is updated on pointer enter as well as motion, allowing a selection to be immediately active when slurp starts. The first commit doesn't change anything, just rearranges some functions in main so I can use them without declaring them elsewhere. The second function does all the changes, by moving some of the motion code into seat_update_selection and calling it in handle_enter as well. PTAL.

[emersion]

LGTM, thanks!

Splitting the changes into two commits definitely made the review easier :)