vc1dec: Use second_field instead of cur_field_type for deciding something

This change was done within the merge commit of the original
VC1 interlaced support in 0edf7ebcd67. Better commit message and
explanation appreciated.
vc1dec: fieldtx is only valid for interlaced frames

the fieldtx_plane is not cleared for interlaced fields

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vc1dec: Don't attempt error concealment on field pictures

This is not implemented and doesnt work.
vc1: implement frame interlaced b-frame header parsing
vc1dec: interlaced B-frames
vc1dec: dont apply the loop filter on fields

Fixes read of uninitialized memory.
vc1dec: Fix destination pointers in context for fields

This replaces a large number of checks for the second field by
fixing the pointers when they are setup

This should also fix I/BI field pictures

Changes checksums for vc1_sa10143, the file becomes slightly closer
to what the reference decoder outputs

Based on "vc1dec: the second field is written wrong to the picture"
         by Sebastian Sandberg <sebastiand.sandberg@gmail.com>
vc1dec: Fix tff == 0 handling in init_block_index()

This fixes several files from VLC ticket5887.
vc1dec: Fix mv_f shuffling - todo: Reindent
vc1dec: Fix mv_f shuffling - todo: Squash?

Avoid a (confusing) memcpy()
Simpler code
Fixes a small number of artifacts in black_screen_VC-1.mkv
and several more artifacts in other videos
vc1dec: factorize srcU/V offseting out
vc1dec: factorize picture pointer selection code
vc1dec: factorize picture pointer selection in vc1_mc_4mv_chroma()
vc1dec: fix current ptr selection in vc1_mc_4mv_chroma()

No sample tried shows a difference.
vc1dec: do not allow field_mode to change after the first header

Fixes out of array accesses.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

TODO: Not sure if this is relevant security-wise for us, but later
commits depend on it.
vc1: simplify code use INIT_LUT()
vc1: make INIT_LUT() self contained - todo: reindent?

factorize variable declarations
vc1dec: redesign the intensity compensation

The existing implementation had little to do with VC1.
This could be implemented by adjusting the reference frames
ithemselfs but that would make frame multi-threading difficult.
vc1dec: Implement intensity compensation for vc1_interp_mc()
vc1: use use_ic instead of shuffling mv_mode around