Cleanup of window coordinate positioning code.

Cleanup of window coordinate positioning code.
Now, text area click input events measure Y from the top of the text
area, excluding the header line if any.

* src/dispnew.c (buffer_posn_from_coords): Assume that X counts from
the start of the text area.

* src/keyboard.c (make_lispy_position): For text area clicks, record Y
pixel position relative to the text area, excluding header line.
Also change X and Y to Lisp_Objects, not pointers; don't return
coordinate values via pointers.  Pass ON_TEXT_AREA coordinate to
buffer_posn_from_coords counting from the start of the text area.
(Fposn_at_x_y, make_lispy_event): Callers changed.

* src/w32term.c (w32_read_socket):
* src/msdos.c (dos_rawgetc):
* src/xterm.c (handle_one_xevent): Likewise.

* src/window.c (coordinates_in_window): Change X and Y to ints rather
than pointers; don't return coordinates via pointers.
(struct check_window_data): Change X and Y from pointers to ints.
(window_from_coordinates): Remove args WX and WY; don't return
coordinates via pointers.
(Fcoordinates_in_window_p, window_from_coordinates):
(check_window_containing, Fwindow_at): Callers changed.
(window_relative_x_coord): New function.

* src/window.h (window_from_coordinates, window_relative_x_coord):
Update prototypes.

* src/xdisp.c (remember_mouse_glyph): Change window_from_coordinates
call.  Use window_relative_x_coord.
(note_mouse_highlight): Change window_from_coordinates call.