Arrange to tell redisplay about changes in overlays.

Arrange to tell redisplay about changes in overlays.
	* xdisp.c (redisplay_region): New function.
	* buffer.c (Fmove_overlay): Call redisplay_region on the areas the
	overlay has enclosed or left.
	(Fdelete_overlay): Call redisplay_region on the area the overlay
	used to occupy.
	(Foverlay_put): Call redisplay_region on the area the overlay now
	occupies; we may have put a face property on it.

	* xdisp.c (redisplay): If we're doing a thorough redisplay (all
	windows on all frames involved), go ahead and flush the GC cache -
	call clear_face_vector.

	* xdisp.c (display_text_line): Apply faces to characters
	according to overlays and text properties; use
	compute_char_face and compute_glyph_face to figure out what
	face to use, and where a new face starts.
	* xterm.c (dumpglyphs): Use the upper bits of the glyphs to decide
	which frame face to use.  Call GLYPH_FOLLOW_ALIASES to make sure
	we're implementing the glyph table properly.  If we're not using
	the default or mode line face, call intern_face to find a display
	face for the frame face selected by the glyph code.  Implement
	underlining.  Remove the `font' argument; we have to derive this
	from the frame and face anyway.  Change all callers.
	* disptab.h (GLYPH_FOLLOW_ALIASES): New macro.