[#610] Fix me.Container positioning

- Reverts the broken inViewport test; use the object geometry instead of object bounding box
- Fixes the me.Entity geometry for world shapes created with Tiled PolyLine objects

Revert "[#569] Fix the disappearing platforms"

This reverts commit 104b6f45a7c366a662582a796a922fde2ec51ad0.

Conflicts:
    src/renderable/container.js