summary: This patch modifies the `curves` filter with new `interp` option
         to let user pick the existing natural cubic spline interpolation
         and the new PCHIP interapolation.

reason:  The natural cubic spline does not impose monotonicity between
         the keypoints. As such, the fitted curve may vary wildly against
         user's intension. The PCHIP interpolation is not as smooth as
         the natural spline but guarantees the monotonicity. Providing
         both options enhances users experience (e.g., reduces the number
         of keypoints to realize the desired curve). See the related bug
         report for the example of an ill-interpolated curve.

alternate solution:
         Both Photoshop and GIMP appear to use monotonic interpolation in
         their curve tools, which were the models for this filter. As
         such, an alternate solution is to drop the natural spline and
         go without the `interp` option.

related bug report: https://trac.ffmpeg.org/ticket/9947 (filed by myself)
Signed-off-by: Takeshi (Kesh) Ikuma <tikuma@hotmail.com>