Since the group element and the clipping path elements are found
on separate iterations, I need to preserve the context of the
group. That is, when a group is found, push that element to the
stack. When the clipping path is found I know it applies to the
current group that is on the top of the stack, and issue a call to
our renderer to begin clipping the text which follows. We don't
know the clipping window until we process the path that is within
the group. We then iterate each text element, and when we get the
group end, we pop the group off the stack, and I know the clipping
path applied to it is done, so I must then call our renderer to
end clipping text.
