Feature: add control on target output area size

[abey79]

Currently, the physical span of hatched's output is determined by the combination of the image resolution and the scaling factor (-s). This is unfortunate because the output's physical dimension is often something the user might want to control.

Specifically, in order to adjust the image scaling factor while keeping the same line density and physical size requires quite the contortion:

# same dimension and density:
vpype hatched -p 1mm input.png
vpype hatched -p 0.5mm -s 0.5 input.png scale 2 2

This would be solved by adding a -a/--area X Y parameter to control the physical output size. In particular, it would allow to freely adjust the scale without impacting the output dimension nor the line density:

# same dimension and density:
vpype hatched -p 1mm -a 10cm 10cm input.png
vpype hatched -s 0.5 -p 1mm -a 10cm 10cm input.png

[abey79]

@mariana-s-fernandes any motivation to tackle this one? ;)

[mariana-s-fernandes]

I can give it a try, but not in the next couple of weeks though :)

[mariana-s-fernandes]

I am starting implementing this. Not sure if I understand it though. Could you pass area and scale simultaneously as input?

[mariana-s-fernandes]

Sorry, I've started this but got derailed. I will pick it up in the future or, if someone else wants to pick it up, check my fork here https://github.com/mariana-s-fernandes/hatched/tree/area-input