Third question on this forum, third time no replies. Starting to think there are stupid questions after all 
Answering for people stumbling upon this question, wondering the same thing. I came a lot closer to a solution using the geomerative library by Richard Marxter. I set a transform matrix with shear and apply it to the shape. I can now compare the new width to the original width and scale accordingly before drawing the shape.
It doesn’t look like it’s 100% correct, not really sure why. Also have some trouble animating it nicely because the shear transform in Geomerative takes vectors instead of the ‘end situation’.
If someone has done something similar, or has some advice on how to do this correctly. Let me know.