Also add parentheses to some lines to make the operator precedence
clearer.

This affected the FATE-tests vsynth*-jpeg2000 and vsynth*-jpeg2000-97
(where * ranges over { 1, 2, 3, _lena }) as well as ticket #7983.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>