noflo-canvas: Implement loading of input into canvas

Previously the canvas port could only be used for purely generative,
which caused non-generative filters to reimplement image load handling,
with incompatible parameter names.
The insta_* filters still need to be updated to reflect this
graphs/insta_1977: Use input data from 'canvas' port

The "insta" graph was a bit too generic, removed.