https://bugs.freedesktop.org/show_bug.cgi?id=13084 demonstrates a
regression where the clip is ignored by the xlib backend whilst
rendering text. Therefore we extend the current test cases to combine
a clip that represents everything clipped out with the rendering of
text. For completeness, we add a new test case that exactly mimics the
sequence of cairo operations that lead to the discovery of this bug -
namely the rendering of text with an empty clip path.
