Selection tool very slow for larger images

[kroppt]

When opening large images, such as those taken by expensive cameras, reaching into 10,000 by 10,000 pixels, it's really slow.
Our current implementation uses a map to keep track of which pixels are currently in use. The problem is that such an image needs 100 MB of space if using a single byte to store that information, assuming no overhead, like in an array.
Perhaps we should consider rectangle selections that don't save all that information, at least as an alternative in big images.