pixman-filter: integral splitting is only needed for triangle filter

Only the triangle is discontinuous at 0. The other filters resemble a
cubic closely enough that Simpsons integration works without
splitting.

Changes by Søren: Rebase without the changes to the integral function,
update comment to match the new code.

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