pixman-filter: Made Gaussian a bit wider

Expanded the size slightly (from ~4.25 to 5) to make the cutoff less
noticable.  Previouly the value at the cutoff was
gaussian_filter(sqrt(2)*3/2) = 0.00626 which is larger than the
difference between 8-bit pixels (1/255 = 0.003921). New cutoff is
gaussian_filter(2.5) = 0.001089 which is smaller.

v11: added some math to commit message
v14: left SIGMA in there
Signed-off-by: Bill Spitzak <spitzak@gmail.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Søren Sandmann <soren.sandmann@gmail.com>