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.
Call NEXT() only once since it got slower due to the overflow condition
introduced in 91b46145.
interp_trilinear: 1462 → 1280 decicycles
interp_tetrahedral: 1188 → 1097 decicycles
Tested on a Core2, GCC 4.8.