rio: make window focus changes deterministic, cleanup wind.c

rio: make window focus changes deterministic, cleanup wind.c

Switching window focus used to be non deterministic
as the current window in focus (Window *input) was set
concurrently while processing window messages such as
Resized and Topped.

This implements a new approach where wcurrent() and
wuncurrent() are responsible for the synchronization
and switch of the input.

It is implemented by sending a Repaint message to the
old input window first, neccesarily waiting until that
window releases the focus and then input is updated
and then a Topped or Reshaped message is send to the
new input window.

Note, that when the whole screen is resized that no
input changes need to happening anymore.