The scene background can now be a gradient or a bitmap image in addition to a static color; use scene.SetBackground(gfx.Gradient) or scene.SetBackground(gfx.Bitmap) for this purpose.

The gfx.Bitmap class and its import/export functions have been exported to Python.

Some cleanup in the OpenGL code to (i) avoid premature rendering before full initialization and (ii) streamline error checking a bit.