FEATURE: generate which doesn't overwrite existing

[andrzejnovak]

I have a minor hassle when adding new tests and running

 --mpl-generate-path=tests/baseline

I overwrite all existing baseline images in my repo. I then have to sort through which are added, because usually the regenerated old tests are still the same visually, but are not identical hash-wise.

I would propose adding a

 --mpl-generate-missing=tests/baseline

Which would only generate new figures if the baseline is mising.