Code gen improvements

[PeterJohnson]

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Code gen is lacking a few features that would make it more useful / more efficient out of the box.

• [ ] Filter contours generated code is inefficient (it performs all computations even if only some of the values are actually compared)
• [ ] Save images is not implemented
• [ ] The hooks for getting intermediate images (e.g. to stream a processing step separately for debugging) don't always work, as some CV operations (contours) modify the source image. A better way of inserting code between processing steps would be very helpful.
• [ ] Individual processing step functions are marked private instead of public so the above can't be worked around (e.g. by writing process() externally) while keeping most of the generated code intact. This would also be useful e.g. if you want to set HSV values externally vs using the GRIP generated values).