rsvg: Add animated SVG support

Submitted by Thibault Saunier @thiblahute

Link to original bug (#796909)

Description

The following pipeline output EOS but no buffer:

gst-validate-1.0 uridecodebin uri=https://gitlab.gnome.org/GNOME/pitivi/uploads/8dd8d9d988b5eb6cc38f871196caac6f/Titel-Tafel3.2_anim.svg  ! imagefreeze ! identity silent=false ! videoconvert ! checksumsink  

It should output the exact animation described in the SVG.