When using fonts circular references are established between the holdover
font caches and the interpreter which need manual intervention via
cairo_script_interpreter_finish() to break.
Add a CairoScript interpreter library and use it to replay the test output
for the CairoScript backend. The library is also used by the currently
standalone Sphinx debugger [git://anongit.freedesktop.org/~ickle/sphinx].
The syntax/operator semantics are not yet finalized, but are expected to
mature before the next stable release.
