add test of linear interpolation

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add cubic interpolation test, did not work out too well

Signed-off-by: Mattias Andrée <maandree@operamail.com>
m + fix cubic interpolation

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