rsvg-handle: take viewport as pointer for get_geometry_for_element()

Vala has trouble with mixing out arguments and stack values.
RsvgRectangle viewport can be passed as const pointer instead.

Apparently the API isn't fixed yet, let's propose a change while it is
still time.

Related to:
GNOME/librsvg#454

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
7 jobs for get_geometry_for_element in 62 minutes and 46 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #295703
fedora test

00:11:35

 
  Lint
manual #295705
allowed to fail manual
clippy
manual #295704
allowed to fail manual
rustfmt
passed #295706
tarball

00:13:33

 
  Cross Distro
passed #295709
debian_i386:testing

00:37:36

passed #295708
allowed to fail
debian:testing

00:21:45

passed #295707
allowed to fail
fedora:rawhide

00:23:20