Started implemented asynchronous Xflow operators. Everything untested.
Added basic (kinda hacked) support for asynchronous Xflow operators.
Implemented proper checks for asynchronous operator execution.
Added noAlloc option for Xflow operator output.

If noAlloc is enabled, no output buffer is allocated and passed to the operator.
Instead, a object with the member "assign" is passed.
The operator can allocate its own buffer/image and assign it via
[output argument].assign = [buffer / image];

See the xflow.bufferSelect operator for an example.