Increase maximum images size

[kba]

Current situation

The upper limit of PIL, our image library, for loading images is 89478485 which is equivalent to ~ 9400x9400 pixels. Large or very high pixel-depth scans, such as of newspapers, maps etc. can be larger than that. Loading a larger image will result in an error to the user.

How it should be

The upper limit should be future-proof, 40k by 40k pixels for a possible image seems a reasonable upper bound.

As we move towards networked components, this setting should be documented to make developers aware that this could be a potential source for DOS attacks by spamming a server with bogus large images.

Steps

• Merge Allow images with up to 40k by 40k pixels, fix #735 core#768
• Release a new version
• Document this (where?)