(help_echo_window): New variable.

(help_echo_window): New variable.
(syms_of_w32term): staticpro it.
(note_mode_line_highlight): Set it.
(XTextExtents16): Removed as there is no equivalent on W32.
(x_compute_glyph_string_overhangs): Incomplete body removed, as
the X way of doing this will not work for W32.
(w32_intersect_rectangles): Removed. Use IntersectRect API call.
(x_draw_image_foreground):  Avoid drawing outside of the clip area
when image doesn't have a mask.
(note_mouse_highlight): Process overlays in the right order of
priority. Set help_echo_window.
(x_draw_bar_cursor): If cursor is on an image, draw a box cursor
because that's more visible for large images.