Allow customizing bitmap handling in wxSVGFileDC.

Provide a built-in alternative for using external files for the bitmaps in
SVG: allow embedding them inside the SVG itself using "data:" URI.

And also allow to define custom handlers to make the behaviour even more
flexible.

Closes #15968.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75981 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775