Remove nearest-int position rounding from SpriteRenderer.

The things that want to be rounded already do this
much earlier (wr.ScreenPxPosition, etc).
Introduce Renderable.BeforeRender().

WorldRenderer.Draw() has been slightly reorganized
to ensure that BeforeRender is called before any
drawing render state (e.g. scissor) has been set.
WIP: FBO Rendering.
Voxel test mod.