Make XYToWindow a screen function

This allows DDXen to override the window picking to account for
native windows not seen by the X server.  The bulk of the picking logic
is exposed as a new helper function, miSpriteTrace().  This function
completes the sprite trace filled out by the caller, and can be set up
to start the search from a given toplevel window.

v2: Leave existing XYToWindow API in place for API compatibility

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>