bug#67794: 30.0.50; mouse-face is not respected on SVG images

[Eli Zaretskii]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: gerd.moellmann@gmail.com,  alan@idiocy.org,  67794@debbugs.gnu.org
> Date: Thu, 14 Dec 2023 10:07:41 +0100
> 
> Would it be possible to lookup_image with the current mouse face at
> glyphs generation time for a string with the mouse-face property and
> then, using this image glyph when showing the mouse face?

I guess it's possible, but it requires changes in how the display code
works:

  . when we generate glyphs, we currently pay no attention to
    mouse-face properties, only to face properties
  . image glyphs will need to store 2 image IDs, not 1, and use the
    appropriate one in each case