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

[Eli Zaretskii]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: alan@idiocy.org,  67794@debbugs.gnu.org
> Date: Wed, 13 Dec 2023 15:13:08 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > It behaves exactly like a face, and has the same attributes, thus the
> > MOUSE_FACE_ID enumeration.  But it is handled specially by the display
> > engine, because it changes the appearance depending on mouse pointer
> > position, without any changes to the mouse-face definition itself.
> 
> Ok and so my next question is: Is mouse highlight handled completely
> outside the « regular » display engine?  If so, I will never ever see a
> lookup_image call with a face_id == MOUSE_FACE_ID, right?

Right.

mouse-highlight works differently than the "regular" display: it
redraws already drawn portions of the screen by (re-)drawing the same
glyphs with different colors.  By contrast, lookup_image is called
when a 'display' property is found during _generation_ of those
glyphs, before showing them for the first time.