SVG inlining code needs to escape `#` characters

This is in the context of librsvg#377 (closed):

GTK's code for icon themes takes the actual SVG data for an icon, and wraps it with extra styling information by generating the following data:

<svg ...>
  <style>
    ... extra styling here ...
  </style>
  <xi:include href="... original SVG encoded as a data: URL ..."/>
</svg>

The master branch of librsvg replaced the C code that parsed data: URLs in a rather loose way, with the data-url Rust crate that does this in a stricter fashion.

GTK creates the data: URL by concatenating these:

• data:text/xml,
• The result of g_markup_escape_text() on the original icon's SVG data.

However, per the data: URL spec, this generates an incorrect data: URL when the original SVG data has a # in it, for example, in a color="#000" attribute. The data: URL decoder thinks that everything past the # is the URL's fragment identifier, and ignores it (data: URLs cannot have fragment identifiers).

GTK needs to escape # as %23 while building the data: URL. (And librsvg needs to handle faulty xi:include content properly, but that's librsvg#377 (closed).)

Addendum: the "wrap an icon's SVG" code is duplicated among gtkicontheme.c and encodesymbolic.c, but the wrapper is slightly different in each. I don't know if they need to be the same; both do need to be fixed to escape # properly.