pixman-filter: Speed up BOX/BOX filter

The convolution of two BOX filters is simply the length of the
interval where both are non-zero, so we can simply return width from
the integral() function because the integration region has already
been restricted to be such that both functions are non-zero on it.

This is both faster and more accurate than doing numerical integration.

This patch is based on one by Bill Spitzak

    https://lists.freedesktop.org/archives/pixman/2016-March/004446.html



with these changes:

- Rebased to not assume any changes in the arguments to integral().

- Dropped the multiplication by scale

- Added more details in the commit message.

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