Multithreaded encoder, late sync loopfilter

Multithreaded encoder, late sync loopfilter

Second shot at this...

Sync with loopfilter thread as late as possible, usually just at the
beginning of next frame encoding. This returns control to application
faster and allows a better multicore scaling.

When PSNR packets are generated the final filtered frame is needed
imediatly so we cannot delay the sync. Same has to be done when
internal frame is previewed.

Change-Id: I64e110c8b224dd967faefffd9c93dd8dbad4a5b5