Image Size in Canvas reverts to 256sq when saving with Sep Image size and not locking aspect ratio

[r-englund]

Steps to reproduce:

1. Add a canvas
2. Check the "Separate Image Size" boolean
3. Uncheck the "Lock Aspect Ratio" boolean
4. Set the "Image Size" to anything but 256x256
5. Save and reload the network (or copy paste the canvas into the same network)
6. Image Size is reseted to 256x256, independent on the canvas size