Split tall images into horizontal chunks

[serialuart]

Currently, when a thread is very long, the render becomes hard to read:

Add a feature to split the image horizontally.

[serialuart]

I've had some time to think about this, and imo the best course of action would actually be to just roll up the post above a specific height (roughly calculated from the height of the individual elements), and re-use the unroll function for those who want to see the whole post regardless.

[serialuart]

Another idea I had related to this - for long posts, we could add a "buffer" page that is just the rendered thread with some minor changes (the page is centered, video embeds actually work, use iframe embeds, etc.) and a link to redirect to the Tumblr post. This way users don't have to log in to see the whole post before the login-wall kicks in. (We could also make this a default option by either setting an URL parameter or through an user preference stored in the browser cookies; FixTweet has something similar for some options, could be cool to add a settings page.)