Layer shell clients may leave the output on which to display a layer
surface up to the compositor. Instead of always putting such surfaces
on the first output use the focused output of the default seat.
Focus was made double-buffered in 96a91fd. However, much of the code
still behaved as if focus was separate from the transaction system.
This commit completes the work started in 96a91fd and ensures that
focus is applied consistently in a single transaction.
When a button is held down and the cursor leaves a surface, events now
continue to be sent to the client. This allows e.g. dragging a scroll
bar from outside the surface.
river is not a library and passing a general purpose allocators around
everywhere does not make sense and leads to ugly code. This does not
prevent us from using local arenas if they are fitting.
