Reimplement zooming to work more generally

[maxdexh]

Allows zooming in past fill-screen, as well as zooming on horizontal pdfs in general.

I could not get zooming out past fit-screen to work well because the image dimensions are converted to cells too early (which rounds the aspect ratio). This should be a non-issue since there is no point in doing so right now.

The zooming code is (in my opinion) also a lot more comprehensible now and has some comments explaining what is going on.

[itsjunetime]

Holy shit this works so well; thank you for figuring this all out.

There are a few small nits, mostly about comments, but overall everything is really good. If you want to take care of the nits later as well, I'm fine with that.

On a similar note, are you interested at all in being a co-maintainer? No pressure either way, but I have really appreciated your fixes recently :)

[maxdexh]

On a similar note, are you interested at all in being a co-maintainer

Thanks so much for the offer! That'd be great :)

[maxdexh]

Oh also I forgot to mention, the early conversion to cells also causes the page to be blurry when zooming in, since we are zooming in on the image, not the pdf. I plan to fix this along with the inability to zoom out in the future, but it will require a larger refactor.