pixman-filter: Correct Simpsons integration

Simpsons uses cubic curve fitting, with 3 samples defining each
cubic. This makes the weights of the samples be in a pattern of
1,4,2,4,2...4,1, and then dividing the result by 3.

The previous code was using weights of 1,2,0,6,0,6...,2,1.

With this fix the integration is accurate enough that the number of
samples could be reduced a lot. Multiples of 12 seem to work best.

v7: Merged with patch to reduce from 128 samples to 16
v9: Changed samples from 16 to 12
v10: Fixed rebase error that made it not compile
v11: minor whitespace change
v14: more whitespace changes

Signed-off-by: Bill Spitzak <spitzak@gmail.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Søren Sandmann <soren.sandmann@gmail.com>