virtual bool BeginScene();
BeginScene begins scene rendering, initializing various render states such as blend, scale9 states needed externally; EndScene completes scene rendering. In Direct3D, this will call BeginScene/EndScene flags on the device. If not called explicitly, these functions will be automatically called from BeginDisplay and EndDisplay. Calling them externally is more efficient if multiple BeginDisplay/EndDisplay blocks will take place, as it optimizes state changes and eliminates queue flush.
Returns true if the scene is rendered successfully and false otherwise.