Feature discussion: determine output canvas size

[RobertMueller2]

Discussed in #110 and #336. Overlaps with #337 and #288.

Annotations can be (partially) outside of the image, there should be a way to save more canvas than just the image.

• Should there be a silent default, e.g. use the outer bounds of all annoations, regardless of current visibility?
• Should there be a way to explicitly set outer bounds? Would this be an improved crop feature?
  • if so, should there be an actual applied crop on top of this?