Implement rendering the game scene on the splash screen

[rozniak]

Enough of rzxe has been implemented and some shell code done that makes me happy to work on some actual 'game' again. As a first step, I want to render the scene as in the test code I originally wrote when parsing the level data.

The splash screen contains a little window where a 'demo' of the game can be tried out. This is a good candidate for testing simple game scene code.

[rozniak]

Depends on rozniak/rzxe#15 for the animation stuff - am currently working on that alongside this.