input: Get rid of grab focus concept

input: Get rid of grab focus concept

This was another complication that we had to have to support the
split between libwayland-server and weston.  Different grabs want to send
events relative to different surfaces at different times.  The default
grab switches between sending coordinates relative to the 'current' surface,
that is the surface the pointer is currently above, or the 'clicked'
surface, in case of an implicit grab.

The grab focus was set by the grab implementation and the core input code
would transform the pointer position to surface relative coordinates for the
grab focus and store in grab->x/y.

Now we can just let the grab implementation transform the pointer
coordinates itself, leaving the implementation free to transform
according to whichever surface it wants.  Or not transform at all if
it doesn't need surface relative coordinates (like the shell move and resize
grabs).