Add Preprocessing

[WillNilges]

I'm seeing hugely better results if I apply preprocessing with OpenCV before passing images to Inkpath. It's definitely not out of scope to add some CV libs and see if we can't have that implemented directly in the plugin.

TODO: Test on my 9-year-old T430s 😂

[WillNilges]

I wonder if I can effectively straighten the image as well (or somehow ask the user to do it manually)

[WillNilges]

I wonder if we could make OpenCV auto-crop by singling out white rectangles in the image and removing the rest of the pixels. Maybe we can also make it auto-straighten by trying to straighten out the edges like we did with the pool table.

[WillNilges]

I'm going to be using some userdata bullshittery to get data from OpenCV to my Lua plugin. From there, I'll go stroke-by-stroke, pushing things onto the Lua stack and passing them to Xournalpp. A possible optimization to make down the line would be to skip the Lua stack altogether and just shunt contours directly to Xournalpp somehow.

[WillNilges]

TODO: build and install OpenCV in the Dockerfile :3

[WillNilges]

Seems like inkpath doesn't like plusses or T's. Hmm.