Manual lens correction

[lilygthb]

at the moment, if a lens is not included in the lensfun database there is no way to apply any correction. This leads to severely distorted images that can be unusable:

There should be a sort of manual control (this is from darktable 5.6)

Maybe the manual option wasn't included in darktable 4 so the code didn't make to Ansel, but I think is very necessary.

[paolo-santucci]

That are the controls for fine tuning the embedded metadata lens correction, not manual correction.

I'm currently working at porting the embedded metadata lens correction method from Darktable to Ansel, but it works only with Sony, Olympus, Fujifilm and DNG.

Also not all the cameras save those metadata in the raw files. For example I had a Sony RX10 III and there's no embedded lens correction in its files.

[lilygthb]

I saw your PR I I thank you a lot for it.
Maybe in the future, for raw files without lens correction metadata, it could be viable to implement a simple Barrell/pinch and vignette correction. Better than nothing...