Rewrite Entity inheritance to use the new Renderer system.

ScreenEntities are now SceneEntities2D, generally. MobileEntity
shape is now VPLANE, which makes it invisible due to projection
since the ColorAffectsChildren flag seems to be broken.

PlayerEntity must now register with the coreInput for key events
directly, as well.
Merge pull request #2 from johkelly/polycode2

Rewrite Entity inheritance to use the new Renderer system.