Support GIMP curves preset files in curves filter

[llogan]

The curves filter can accept Photoshop curves files, but not GIMP curves files.

Attached is a GIMP curves file that increases contrast. Made with GIMP 2.8.14.

[llogan]

GIMP curves preset file

[Ilya87]

If only I could write I a parser!

1 - There are two types of curves in Gimp - Free (drawed by hand) and Smooth.
1.1 - This type stated in (curve (curve-type free) or (curve (curve-type smooth) section for every color channel.
2 - Only 17 points on channel can be set in Smooth mode.
3 - The order of color channels in the file: value, red, green, blue, alpha.
4 - If curve-type smooth is set, then coordinates of points must be read from section (points 34 from the beginning, negative values must be ignored, positive values follow in pair (x y) correspond to the ffmpeg coordinates x/y. -1.000000 0.240854 0.139194 -1.000000 means 0.240854/0.139194 for ffmpeg, 0.045714 0.045249 0.091429 0.095023 means 0.045714/0.045249 0.091429/0.095023 for ffmpeg.
5 - If curve-type free is set, then coordinates of points must be read from section (samples 256. There are only y coordinates stated, so x coordinate is calculated by the following formula: x1=0, x2=1/255, x3=2/255, x4=3/255,...,x256=1
6 - All points coordinates pairs follow strictly (for both curve types) x coordinate ascending order.