Encoder loopfilter running in its own thread

Encoder loopfilter running in its own thread

In multithreaded mode the loopfilter is running in its own thread (filter level
calculation and frame filtering). Filtering is mostly done in parallel with the
bitstream packing. Before starting the packing the loopfilter level has
to be calculated. Also any needed reference frame copying is done in the
filter thread.

Currently the encoder will create n+1 threads, where n > 1 is the number of
threads specified by application  and 1 is the extra filter thread. With n = 1
the encoder runs in single thread mode. There will never be more than n threads
running concurrently.

Change-Id: I4fb29b559a40275d6d3babb8727245c40fba931b