rsvg_cairo_render_new(): Initialize ->bbox to the identity affine
rsvg_cairo_render_new(): Initialize ->bbox to the identity affine

Otherwise, when filters cause rsvg_get_surface_of_node() to be called,
that function will create a RsvgCairoRender with an all-zeros bbox and
bbox.affine.

This all-zeros affine causes cairo_matrix_invert() inside
RsvbBbox::clip()/insert() to fail.  In the C version it failed silently,
since it ignored failures from cairo_matrix_invert().  The Rust version
panic()s when a matrix can't be inverted.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg