Separation and DeviceN Colorspaces

MuPDF Colorspaces are extensible, so specific document handlers can implement their own new spaces. A good example of this is how PDF implements Separation and DeviceN colorspaces.

These are special spaces which represent arbitrary sets of 1 or more colorants. These can either be mapped down to `equivalent' colors in a more standard space, or (depending on the capabilities of the underlying device) processed in their raw form.