lavc: add a librsvg rasterization library wrapper

Enables rendering of SVG images. This is possible since SVG images
still contain and specify the dimensions in pixels to which they've
been drawn to and thus enable browsers to display them without any
external data. Users can still override and generate images with
arbitrary resolutions.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>
