sw_engine: retain the compositor cache memory

The compositor memory is likely to be reused in the next frame.
To enhance performance, it is advisable to retain this memory by default.

We may consider introducing a cache policy interface in the Initializer.
This would allow users to manage the Canvas memory more effectively.

Anyhow, this improves the Lottie example performance by 10%

src/renderer/sw_engine/tvgSwRenderer.cpp

@@ -475,7 +475,6 @@ bool SwRenderer::postRender()
     }
     tasks.clear();
 
-    clearCompositors();
     return true;
 }