Don't shuffle around Renderable by value.
Move all remaining rendering to render device.

All rendering to screen is now done via the RenderDevice interface.

This means there is now full OpenGL rendering support.

Some caveats:

- OpenGL rendering forces animated tiles. Who doesn't want that anyway?

- the texture caching for animations does not work yet, so these
  textures are createt/destroyed on the fly by the OpenGL render device.
	This has no impact on the SDL blit render device. Also it does not seem
	to have a big impact on the OpenGL rendering performance.  It would
	still be nice to do things as fast as we can, though.

- VSync in OpenGL mode only works in full screen and only if the driver
  is forced to VSync. To get VSync in windowed mode/by configuration
	requires dealing with extensions. That is a bit of a pain for
	portability. Separating the the game state update from the rendering
	might help as well. Will be looked into with low priority.

- Some throw-away textures, e.g. for WidgetScrollBox, might become too
  large for less powerful OpenGL implementations.