(#334): Preserve the cairo_t's transformation matrix during rendering

Part of the API contract is that calls to rsvg_handle_render*() will
not modify the passed cairo_t.  However, internally we were doing
cairo_save() only until after first creating the DrawingCtx,
and that operation modifies the cairo_t.

Fixes GNOME/librsvg#334
3 jobs for <span class="ref-name">preserve-matrix-during-rendering</span> in 8 minutes and 6 seconds (queued for 13 minutes and 33 seconds)
Status Job ID Name Coverage
  Test
passed #120787
fedora test

00:07:43

 
  Lint
manual #120789
allowed to fail manual
clippy
passed #120788
rustfmt

00:00:23