Zoomed in pdf is blurry

[maxdexh]

The rendering to fit-screen format causes the page to be blurry when zooming in past fill-screen, since we are zooming in on a rendered image, not the original.

This limitation also makes it so we cannot implement zooming out past fit-screen without falling victim to severe rounding errors.

See #121 (comment)

[maxdexh]

The way this is handled by firefox and zathura is that it zooms into the existing render, which is blurry, but asynchronously requests a new (local) higher-resolution render. The zoomed-in render is shown once it is done. Zooming in further or moving the view also initially zooms in on that, as long as the view stays in bounds of the local render. Leaving the local render, e.g. by zooming out, discards it, replacing it with the (potentially blurry) full render again.

[itsjunetime]

Yeah, I think this would be good to do - I think we probably want the re-renders to be somewhat granular, e.g. every 5 zoom 'levels' or smth requests a re-render (instead of anytime the document zoom level is increased at all).

Do we want to keep around the original fit-screen render for when they zoom back out, or just re-render once they zoom out? Could save a bit of memory to not hold onto it?