Expose more of the game loop internals to the user

[17cupsofcoffee]

It'd be nice if it were possible for users to opt out of the standard game loop and write their own, if they so desire.

Most of the groundwork for this is already there - the missing bits are:

• Access to the timing APIs (not strictly neccecary, as it's easy to implement yourself)
• A way to force the engine to poll for SDL events
• Access to the API that updates input state
• A way to show/hide the window

The implementation of the actual looping needs to provided by Tetra, so that we can abstract over blocking/non-blocking APIs. I think this will probably boil down to the user providing a closure that represents one iteration of the game loop, and then scheduling that closure will be handled by the engine.

[17cupsofcoffee]

#122 might assist in this.

[17cupsofcoffee]

All of the functionality listed above is now exposed via the (private) platform API - just need to figure out a way of exposing that publicly.