Fix special characters handling in wxSVGFileDC::DrawText().

Special XML characters need to be quoted before being saved in an SVG file
(which is also an XML file).

Closes #15602.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75068 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775