Fix display of mouse-highlight produced by overlapping overlays
* src/xfaces.c (face_at_buffer_position): If called to find the
mouse-face, only consider the highest-priority source for that
face, and ignore the rest. Previously, all the mouse-face
definitions at POS were merged in that case.
* src/xdisp.c (note_mouse_highlight): Record the overlay that
specifies mouse-face _after_ clearing the info about the previous
overlay, so as not to clear the information about the just-recorded
overlay. (Bug#25906)