Mouse highlighting and fonts

[Chong Yidong <cyd@stupidchicken.com>]

With the current CVS code, the mouse highlight preserves the size of the
underlying face, which avoids the meta-stability problem I mentioned a
few weeks ago.  However, mouse highlighting of the glyph background
still doesn't work right when the underlying font size varies across the
highlighted region.  A test case:

(progn
  (switch-to-buffer "*Test*")
  (erase-buffer)
  (insert "aaaaaaaaaaaaaaaa")
  (put-text-property 1 6
		     'face (list :foreground "cyan"
				 :background "dark blue"
				 :height 2.5))
  (put-text-property 6 (point-max)
		     'face (list :foreground "violet"
				 :background "dark red"))
  (put-text-property 3 8
		     'mouse-face (list :background "green")))