Problem:
Multigrid UIs have to find out which window to send the input by using
the Nvim focus rules, which are not fully documented.
Furthermore,`getmousepos()` has several problems when multigrid is
enabled, with the main one being that screenrow and screencol are window
relative instead of screen relative, due to the fact that the UI don't
send any absolute coordinates.
Solution:
Allow passing 0 as grid to `nvim_input_mouse`, with absolute
coordinates, which lets nvim determine the actual window to send the
mouse input to. This works as long as nvim is in charge of the window
positioning. If the UI repositions or resizes the windows, it can still
pass the grid it determines like before.