Possible to place elements far outside of scene bounds in-game (weirdness)

[Wuzzy2]

Related to #92.
With some trickery, it is possible to place some elements outside of the scene bounds. There is a lot of weirdness regarding the scene bounds:

• You can place movable objects partly outside of the left bound; you just have to grab the object at the right part of it and move it there. But if you start the simulation, the object instantly gets pushed into bounds.
• You can't place any objects out of the lower bounds. That's OK, I guess, since the lower bound works as an invisible floor.
• You can place objects out of the right bounds. You can even place them far, far away by scaling the window to make it very small in height but very large in width. All the blank space you'll see is probably out of scene bounds but the player can happily place stuff into it
• It is also possible to place stuff out of the lower bounds, you can use the same trick as with the left bound

I don't know if it is intentional but it could make level design a bit trickier. It may also be a potential source of too-easy solutions.
Also, it may be a bit unfair to allow such stuff since players with higher resolutions could have an advantage.
Important: I am only talking about in-game placement; I think it is okay to be able place out of bounds in the editor.

Tested in 055e30d.

[kaa-ching]

I'm getting a headache figuring out how to solve the top and right borders...