Handling Long Documents

[jchilds0]

Currently loading large documents can cause issues due to overflows in cairo, reported in #449, #365, #253, #200 and #65. Instead of using GtkGrid, either

1. Create subsurfaces (for each page) of a single surface using cairo_surface_create_for_rectangle and draw as many pages as can fit to this surface. Then move the pages around, changing which ones are drawn and where, on this surface as the user scrolls, or
2. Limit the number of pages in the grid, changing which pages are contained in the grid as the user scrolls,

This issue is for any comments since this is not a small change to make, which one seems like the better option and would this resolve those issues.