(note_mouse_highlight) <help-echo>: When looking for

(note_mouse_highlight) <help-echo>: When looking for
help-echo for a string which doesn't have help-echo itself, use
get-char-property to obtain the help-echo from the string's buffer
(note_mouse_highlight) <mouse-face>: When on a string that doesn't
have mouse-face, look for mouse-face ``under'' the string for
mouse-face from an overlay.
(fast_find_position): Add parameter STOP.  In the final row,
stop before glyphs having STOP as object.  Don't consider
glyphs that are not from a buffer.