One think Chalk can not handle are really big files. A 125MB tif file is
loading into memory in one go, making Chalk take about 289MB of memory
-- with no image loaded, Chalk takes a more reasonable 41MB. And during
loading, Chalk needs 800MB, and that's a bit much.

Since my employer has paid to upgrade my laptop to 1GB of memory, that's
not a problem for me :-). But it's the principle of the thing. Besides,
loading the file does take a few minutes to convert. And you don't want
to do a quick sharpen on a file of this size.

Now it has never been a design goal to handle really enormous images, vips
does that a lot better than any general purpose paint app can ever aspire
to, but we should give some thought to our way of handling big images.

The problem is layered: it's not enough to have lazy loading of chalk
files (although we should store, when an image gets above a certain
size, an .png file of the rendered image that we can quickly load and
display, and then we should only load the tiles we actually need for
editing/rerendering from the file), but we should also be able to do the
same with tiled tiffs and exr images that support random access loading.

This must be done somewhere in the datamanager code. If an image reader
(whether it's a filter or Chalk's own file format reader) indicates that
it supports random access, then a file handle and a the reader object
is passed to the tile manager.

The tile manager either does nothing until it gets a request for a
certain chunk of data (through one of the read functions or the creation
of an read/write iterator), and only then it starts reading and decoding
image data. Or, the tile manager starts a background thread and begins
converting the alien image data to Chalk tiles, carefully caching them
in a temp file.

Maybe we could even devise an algorithm to delete unused tiles from
memory and cache them on disk; same with undo information.

