rendernodeparser: Parse cairo script

rendernodeparser: Parse cairo script

Use cairo-script-interpreter to parse the scripts that generate cairo
nodes.

This requires libcairoscriptinterpreter.so to work properly, but if
it isn't found we disable this (unimportant for normal functioning)
code and just emits a parser warning.
The testsuite requires it however or it will fail.

A new test is included that tests all of this.