GUI: Automatic canvas scale with size

[daose]

Canvas scale (controllable by mouse wheel) should automatically set to a nice value when the width/height changes.

i.e. for 64x64 we have 8x scale, but when we change to 512x512 the canvas is unbelievably huge now, scale should go back to 1x to adjust

• aside: should implement locking width/height aspect ratio when changing values