This includes a new test, nested_masks in
librsvg_crate/tests/primitives.rs, which captures the parts that were
broken in 761175-recursive-masks.svg.

The problem is that masks were always drawn *outside* of the nested
cr.  The mask was drawn to a temporary surface, but the
DrawingCtx was not aware that the mask was using a temporary cr for
that surface.  So, the test for the cr_stack depth in
CompositingAffines::new() wasn't aware of that temporary cr.

We need to unify the "draw stuff to a temporary surface" abstraction.

765 pass, 1 fail