Reduce mem copies in encoder loopfilter level picking

Reduce mem copies in encoder loopfilter level picking

Do the test filtering in the existing backup frame buffer instead of
the original. Copy the original data into extra buffer before doing
the  filtering. This way there is no need to restore the original
unfiltered  frame at the end of level picking process.

This came up in some discussions with Johann. Thanks!

Change-Id: I495f4301d983854673276c34ec0ddf9a9d622122