bgo#621088: rsvg_cairo_clip_render_new(): Don't leave uninitialized fields in the parent RsvgCairoRender

A text object used as a clip path caused librsvg to crash.  The reason
is that when we create an RsvgCairoClipRender, we were leaving an
all-zeros clip_render->super.bbox field.

This caused two things:

1. An all-zeros RsvgBbox is invalid; it should be "virgin" or actually
have a valid affine.

2. We were not preserving the bbox from the parent rendering context,
so the clipped objects would not know their viewport size.  Now we
copy the original cairo_render->bbox to our derived
clip_render->super.bbox, and start afresh with an empty bb_stack
inside the clip_render.