Improve the scene start logic

[samme]

• Scenes that are LOADING should ignore any start operations
• Scenes that are SLEEPING should be shut down before starting (just like PAUSED scenes)

These changes would avoid the problem where a scene's create() method is called twice, usually duplicating things.

Examples: https://codepen.io/samme/pen/rNYyxyQ

[photonstorm]

Agreed and merged!