bug#36315: 27.0.50; SVG transparency handling is inaccurate

[Eli Zaretskii <eliz@gnu.org>]

> From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> Cc: 36315@debbugs.gnu.org
> 
> An alternative way would be to use rsvg_handle_render_cairo, which is
> recommended by librsvg, and let it blend with the background color.
> 
> Patch attached.  Note that this does not require --with-cairo.
> Raising the required version of librsvg to 2.14 is not a problem, as
> we are already using rsvg_handle_get_dimensions that requires that
> version.  Is Windows librsvg DLL compiled with libcairo?

librsvg on Windows is indeed build with libcairo, but if we don't get
the --with-cairo option, we don't probe for the necessary functions,
so at least theoretically we could have librsvg without Cairo.

The patch looks quite large.  Do we gain anything significant, apart
of the appraisal of librsvg developers?

I've built the patch on Windows (you forgot cairo_surface_destroy, so
I needed to add it), but the result is strange, or maybe I don't
understand what is expected.  I don't see any rectangle of color
#f00000, I see the entire frame with black background, and a few
characters in other colors.

Thanks.