Reduce partial frame copy in encoder's pick_filter_level_fast

Reduce partial frame copy in encoder's pick_filter_level_fast

The partial frame copy function used to copy an extra 8 lines above
and  below. The partial frame filtering can only modify 3 pixel rows
above the partial frame. Reduce copy to bare minimum needed, which is
4 lines, so that partial filtering on copied frame is possible.

Define the "magic" fraction number for partial filtering in
loopfilter.h .

Change-Id: I4791ffc541b6884b12759a0d0714a8faf16147ec