Added apply_curve function.

| method object apply_curve( array(int(0..255)) curve_r,
|                            array(int(0..255)) curve_g,
|                            array(int(0..255)) curve_b )
| method object apply_curve( array(int(0..255)) curve )
| method object apply_curve( string channel, array(int(0..255)) curve )
|
|  Apply a lookup-table on all pixels in an image.
|  If only one curve is passed, use the same curve for red, green and blue.
|  If a channel is specified, the curve is only applied to the
|  specified channel.
|
|  returns a new image object
|
|  arg array(int(0..255)) curve_r
|  arg array(int(0..255)) curve_g
|  arg array(int(0..255)) curve_b
|  arg array(int(0..255)) curve
|      An array with 256 elements, each between 0 and 255.
|      It is used as a look-up table, if the pixel value is 2 and
|      curve[2] is 10, the new pixel value will be 10.
|
|  arg string channel
|      one of "red", "green", "blue", "value", "saturation" and "hue".
|
|  see also: gamma, `*, modify_by_intensity

Rev: src/modules/Image/image.c:1.165