Avoid Buffer Clearing

Clear only buffers that need to be cleared. 
 
 
 
 Color buffer: The color buffer does not have to be cleared if the scene is known to cover all of it. For instance, this is the case if the background image or a sky map covers the whole surface. 
 Stencil buffer: Only needs to be cleared if it is actually used. 
 Depth buffer: If depth test is enabled.