Version 2.51.0

There are many changes in this development release!  This experimental
release is meant for early testing, with the understanding that some
things may be broken, especially since we have large changes to the
way librsvg's artifacts are built.

The biggest user-visible change is that rsvg-convert has been ported
to Rust (Sven Neumann, Paolo Borelli).

Please file bugs at https://gitlab.gnome.org/GNOME/librsvg/-/issues/new
if these changes break your scripts; they are experimental for the
2.51.x development series:

- #646 - rsvg-convert's default DPI is now 96, to better match W3C
  standards.  It was 90 before for historical reasons.  We can change
  this back to 90 if it breaks too many scripts.  You can use the
  options "--dpi-x=90 --dpi-y=90" to restore the old behavior.

- rsvg-convert's --export-id (-i) option should now have more useful
  behavior.  This extracts a particular element from the SVG document
  and renders it scaled to the size specified by the --width/--height
  arguments, or to the pixel size of the element as if it had no
  transformations applied.

- rsvg-convert no longer supports the "xml" or "recording" output
  formats.  These are useful only for debugging Cairo, not for general
  usage.

Known missing features, which should be restored for the stable release:

- rsvg-convert does not allow stdin/stdout streams on Windows; this is
  being tracked in issue #676.

The following are changes related to porting to Rust; they are not
necessarily user-visible, but important for developers of the library:

- There is no leftover C code in the library; all of the
  implementation and the publically-visible symbols are defined in the
  Rust code.  The remaining .h files are all public and do not reflect
  any .c code.  (Sven Neumann)

- The test suite is now ported to Rust.  The only remaining tests in C
  are for the C API itself (tests/api.c).  With the test suite in
  Rust, the tests are automatically run in parallel across CPU cores,
  making test runs much faster.  (Sven Neumann, Dunja Lalic)

- "cargo build", "cargo test" should now work without running
  autotools first.

- The C API is now implemented on top of the public Rust API, without
  special hooks into the library's internals.

The following APIs are deprecated but still available:

- rsvg_handle_render_cairo() - use rsvg_handle_render_document() instead.

- rsvg_handle_render_cairo_sub() - use rsvg_handle_render_layer() or
  rsvg_handle_render_element() depending on what you want to do.

- These deprecations are because the new APIs conform with the web
  world's view of how SVGs should be positioned and scaled in
  surrounding content.  Whereas the old APIs were about rendering SVGs
  to whatever current transformation matrix a Cairo context may
  contain, the new APIs take a rectangular viewport and librsvg
  automatically scales the SVG document to fit in it.  The "natural
  sizing" that was implicit in the deprecated APIs is now explicitly
  documented, and available through the new API
  rsvg_handle_get_intrinsic_size_in_pixels().

New APIs:

- rsvg_handle_get_intrinsic_size_in_pixels() converts an SVG
  document's intrinsic dimensions to pixels, i.e. transforms the SVG
  document's "width" and "height" attributes to CSS pixels.

New features and bug fixes: