Fix mouse pointer avoidance in some configurations

Fix mouse pointer avoidance in some configurations

* lisp/avoid.el (mouse-avoidance-random-shape): Ignore invisible
pointer.
(mouse-avoidance-ignore-p): Adjust for interprogram DND.
* lisp/term/x-win.el (x-pointer-invisible): Make value larger
than the maximum possible glyph.

* src/xfns.c (x_set_mouse_color): Use function that handles
`x-pointer-invisible' too.
* src/xterm.c (x_create_font_cursor): New function.  Handle
invisible cursors.
* src/xterm.h: Update prototypes.