Read at most 1 KiB or 50 lines of a file and render the attributed string in a bitmap context.
The theme is set to Mac Classic to ensure a white background for best look when the previews are used as icons in Finder.
The user’s default fixed width font in size 4 is used. With this font size only around 7 lines are shown, so we need to make it smaller, but we probably want to scale down the result to allow fractional line heights.
A lot of code is copy/pasted from the QuickLook preview generator.
This was suggested in pull request #1226. Ideally we would make the limit flexible, e.g. see how much we can generate in 1-2 seconds, but that is a non-trivial change, so the fixed limit will do for now, and while truncated data is suboptimal, it’s better than being killed due to QL manager’s timeout.
