Fix a 64-bit segfault in kdenlive

Fix a 64-bit segfault in kdenlive

To reproduce:
 - create a new project
 - create a color clip
 - add clip to timeline
 - set an in point on the clip
 - add the box blur effect

The segfault happens because we take the negative of an unsigned
integer. This works out to a signed 32 bit value on a 64 bit
platform, which causes the rgb array bounds to be exceeded.

Signed-off-by: Ray Lehtiniemi <rayl@mail.com>