Log Message:
librsvg: update to 2.48.3.

Version 2.48.3

- Librsvg now requires glib 2.50.0 or later.

- rsvg-convert's --background-color option now accepts colors with
  transparency like #rrggbbaa, rgba(...), hsla(...); the transparency
  value was ignored before (Hao Guan).

- #584 - Fix arithmetic overflow when rendering to large GdkPixbufs.

- #582 - Make librsvg build with Rust 1.39 again.

- #581 - Don't require upcalls into C code for Debian powerpc.