Libav, for some reason, merged this as a public API function. This will
aid in future merges.

A define is left for backwards compat, just in case some person
used it, since it is in a public header.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

FFDIFFSIGN was created explicitly for this purpose, since the common
return a - b idiom is unsafe regarding overflow on signed integers. It
optimizes to branchless code on common compilers.

FFDIFFSIGN also has the subjective benefit of being easier to read due
to lack of ternary operators.

Tested with FATE.

Things not covered by this are unsigned integers, for which overflows
are well defined, and also places where overflow is clearly impossible,
e.g an instance where the a - b was being done on 24 bit values.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

filter_slice calls qsort, so qsort is in a performance critical
position. AV_QSORT is substantially faster due to the inlining of the
comparison callback. Thus, the increase in performance is worth the
increase in binary size.

Sample benchmark (x86-64, Haswell, GNU/Linux),
filter-removegrain-mode-02 (from FATE)
new:
  24060 decicycles in qsort,       1 runs,      0 skips
  15690 decicycles in qsort,       2 runs,      0 skips
   9307 decicycles in qsort,       4 runs,      0 skips
   5572 decicycles in qsort,       8 runs,      0 skips
   3485 decicycles in qsort,      16 runs,      0 skips
   2517 decicycles in qsort,      32 runs,      0 skips
   1979 decicycles in qsort,      64 runs,      0 skips
   1911 decicycles in qsort,     128 runs,      0 skips
   1568 decicycles in qsort,     256 runs,      0 skips
   1596 decicycles in qsort,     512 runs,      0 skips
   1614 decicycles in qsort,    1024 runs,      0 skips
   1874 decicycles in qsort,    2046 runs,      2 skips
   2186 decicycles in qsort,    4094 runs,      2 skips

old:
 246960 decicycles in qsort,       1 runs,      0 skips
 135765 decicycles in qsort,       2 runs,      0 skips
  70920 decicycles in qsort,       4 runs,      0 skips
  37710 decicycles in qsort,       8 runs,      0 skips
  20831 decicycles in qsort,      16 runs,      0 skips
  12225 decicycles in qsort,      32 runs,      0 skips
   8083 decicycles in qsort,      64 runs,      0 skips
   6270 decicycles in qsort,     128 runs,      0 skips
   5321 decicycles in qsort,     256 runs,      0 skips
   4860 decicycles in qsort,     512 runs,      0 skips
   4424 decicycles in qsort,    1024 runs,      0 skips
   4191 decicycles in qsort,    2046 runs,      2 skips
   4934 decicycles in qsort,    4094 runs,      2 skips

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

Speed of all modes increased by a factor between 7.4 and 19.8 largely depending
on whether bytes are unpacked into words.  Modes 2, 3, and 4 have been sped-up
by a factor of 43 (thanks quick sort!)

All modes are available on x86_64 but only modes 1, 10, 11, 12, 13, 14, 19, 20,
21, and 22 are available on x86 due to the number of SIMD registers used.

With a contribution from James Almer <jamrial@gmail.com>

This is how original filter behaves.

Signed-off-by: Paul B Mahol <onemda@gmail.com>

Signed-off-by: Paul B Mahol <onemda@gmail.com>