snapshot: Fix push_repeat () with empty child bounds

snapshot: Fix push_repeat () with empty child bounds

The logic would confuse empty child bounds (in which case nothing should
get rendered) with NULL child bounds (in which case the child node's own
bounds should get used). In fact, if the child bounds are empty, we can
discard the descendant render nodes completely, getting a nice little
optimization.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>