dsputil: Make square put/avg_pixels functions local to h264qpel

Put a copy of the 8bit functions only in dsputil, where they are
used for some other things (e.g. mpeg4qpel, mspel, cavsqpel).

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We could perhaps also try to share specifically the 8bit functions
from h264qpel between it and the others, but that will be slightly
more complicated. H264qpel already had these functions, so we can
simply remove the duplicates.
h264: Clear the mb members via memset instead of using dsputil
h264: Integrate clear_blocks calls with IDCT

The non-intra-pcm branch in hl_decode_mb (simple, 8bpp) goes from 700
to 672 cycles, and the complete loop of decode_mb_cabac and hl_decode_mb
(in the decode_slice loop) goes from 1759 to 1733 cycles on the clip
tested (cathedral), i.e. almost 30 cycles per mb faster.
h264: Only initialize dsputil if error resilience is enabled

It is only used for error resilience. This allows building the
h264 decoder without dsputil, if error resilience is disabled.
dsputil: Make dsputil selectable
configure: Use a different variable name in push/popvar

The variable name 'var' is commonly used to iterate through arguments
in other functions. When the pushvar function internally uses the
variable 'var', it makes pushing/popping the variable 'var' not
work as intended.
configure: Don't do enable_deep_weak on disabled variables

This avoids cases where configure tries to weakly enable an item
which actually is disabled, ending up still enabling dependencies
of the item which itself is only enabled weakly.

More concretely, the h264 decoder suggests error resilience, which
is then enabled weakly (unless manually disabled). Previously,
dsputil, which is a dependency of error resilience, was enabled
even if error resilience wasn't enabled in the end.
vp3: bfin: Transpose the IDCTs

While this change isn't bitexact, the IDCTs weren't bitexact to
start with either.

This simplifies decoupling vp3 from dsputil.
vp3/x86: Use full transpose for all IDCTs

This way, the special IDCT permutations are no longer needed. This
is similar to how H264 does it, and removes the dsputil dependency
imposed by the scantable code.
hpeldsp: Add half-pel functions (currently copies of dsputil)
vp3: Use hpeldsp instead of dsputil for half-pel functions

This makes vp3 independent of dsputil.
vp56: Use hpeldsp instead of dsputil for half-pel functions

This makes vp5 and vp6 independent of dsputil.
indeo3: Use hpeldsp instead of dsputil for half-pel functions

This makes indeo3 independent of dsputil.
bink: Use hpeldsp instead of dsputil for half-pel functions
interplayvideo: Use hpeldsp instead of dsputil for half-pel functions

This makes interplayvideo independent of dsputil.
mimic: Use hpeldsp instead of dsputil for half-pel functions
svq1: Use hpeldsp instead of dsputil for half-pel functions

This makes svq1 independent of dsputil.
mpegvideo: Use hpeldsp instead of dsputil for half-pel functions

This also converts vc1, since that is mpegvideo-based.
svq3: Use hpeldsp instead of dsputil for half-pel functions
svq1enc: Use hpeldsp instead of dsputil for half-pel functions