h264: make slice threading work with deblocking_filter=1

In such a case, decode the MBs in parallel without the loop filter, then
execute the filter serially.

The ref2frm array was previously moved to H264SliceContext. That was
incorrect, since it applies to all the slices and should properly be in
H264Context (it did not actually break decoding, since this distinction
only becomes relevant with slice threading and deblocking_filter=1,
which was not implemented before this commit). The ref2frm array is thus
moved back to H264Context.
