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>