avfilter/lut3d: fix channel order in Iridas format parsing.

According to http://doc.iridas.com/index.php?title=LUT_Formats

    for 3D textures, the data is also RGB, and ordered in such a way
    that the red coordinate changes fastest, then the green coordinate,
    and finally, the blue coordinate changes slowest:

Fixes Ticket #3236.

libavfilter/vf_lut3d.c

@@ -273,7 +273,7 @@ static int parse_cube(AVFilterContext *ctx, FILE *f)
             for (k = 0; k < size; k++) {
                 for (j = 0; j < size; j++) {
                     for (i = 0; i < size; i++) {
-                        struct rgbvec *vec = &lut3d->lut[k][j][i];
+                        struct rgbvec *vec = &lut3d->lut[i][j][k];
 
                         do {
                             NEXT_LINE(0);