pixman-filter: reduce amount of malloc/free/memcpy to generate filter

Rearranged so that the entire block of memory for the filter pair
is allocated first, and then filled in. Previous version allocated
and freed two temporary buffers for each filter and did an extra
memcpy.

v8: small refactor to remove the filter_width function

v10: Restored filter_width function but with arguments changed to
     match later patches

v11: Removed unused arg and pointer from filter_width function
     Whitespace fixes.

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