package/libsvg: new package

Libsvg provides a library for parsing SVG content in files or buffers.
Libsvg does not do any rendering, but instead provides a function-based
interface that can be used by various rendering engines.

[Peter: drop libxml2 conditional]
Signed-off-by: Carsten Schoenert <c.schoenert@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>