Write migration docs from render_cairo*
With the introduction of
rsvg_handle_get_intrinsic_size_in_pixels, the functions
rsvg_handle_render_cairo_sub are deprecated, since they can be implemented in terms of the former one plus one of the new rendering functions that take a viewport.
Since 24b89b4d the old rendering functions are actually marked as deprecated, but are still available in the library.
The documentation should have examples on how to migrate from the old API to the new one, and include recommendations on how to test that SVG assets have an intrinsic size.