Set scene to a minimum size.

[farling42]

When a scene is created, it's dimensions are set to the size of the image.

If a small image is uploaded, then the scene might be too small to be usable.

While the user can manually change the scene size, any map pins will then be in the wrong position.

Allow a minimum scene size; and an image size will be set to a power of 2 bigger if it is smaller than the minimum size.