Hold the last rendered buffer back and don't release it if we don't have anything in queue

Signed-off-by: Simon Busch <morphis@gravedo.de>
Remove debug messages from rendering code

Signed-off-by: Simon Busch <morphis@gravedo.de>
Cancel all incoming buffers when window or it's parent are not visible

Signed-off-by: Simon Busch <morphis@gravedo.de>