add monotone cubic interpolation

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m

Signed-off-by: Mattias Andrée <maandree@operamail.com>
remove unnecessary halo elimination

Signed-off-by: Mattias Andrée <maandree@operamail.com>
document monotonicly_cubicly_interpolate_ramp

Signed-off-by: Mattias Andrée <maandree@operamail.com>
demo monotonicly_cubicly_interpolate_ramp

Signed-off-by: Mattias Andrée <maandree@operamail.com>