librsvg: update to 2.46.3.

Version 2.46.3

- #516 - Fix inadvertent bump in the minimum Rust version required.  The
  string_cache crate is now pinned to a version that still works on
  Rust 1.34.  (Jordan Petridis)

- #521 - Compute geometries correctly if there is a viewBox attribute.

- #308 - Fix stack exhaustion with circular references in <use> elements.

- Consistently use the LGPL 2.1 wherever it is mentioned.

Version 2.46.2

- Patterns and gradients reused across more than one element will only
  get resolved once now; this should make things marginally faster for
  patterns or gradients with fallbacks.

- #515 - Librsvg now has limits on the number of loaded XML elements,
  and the number of referenced elements within an SVG document.  This
  is to mitigate malicious SVGs which try to consume all memory, and
  those which try to consume an exponential amount of CPU time.

Version 2.46.1
- #506 - Fix empty patterns which reference a fallback pattern with
  children.

(wiz)