Lyndon Brown authored Apr 29, 2021 and Jean-Baptiste Kempf committed May 10, 2021

the condition check with the comment "Ignore aligment if a position is
given for video filter" is meant to force absolute mode if positive x/y
values are given (in video filtering mode), causing a separate condition
block to skip calculations for x/y. (it seems x/y offsets from relative
positions are not supported in video filtering mode).

problematically though, the condition check set the position value to
`0` instead of `-1`, and the x/y calculation block condition checked
for a non-zero value, which meant that:
 - center,center relative positioning was not possible (pos=0).
 - if position was set to `-1` (as per default) to signal absolute, and
   x/y were not both positive, the x/y calculation block would set about
   trying to calculate x/y based upon incorrectly reading alignment flags
   from the `-1` position value, thus calculating a bogus position.

this commit fixes these issues by correctly signalling and checking for
absolute mode.