Fixed loading of multilayered PSD files (including 16-bit ones)

This patch does two things:

1) Fixes the order of the saved channel planes. The indexes of the
planes array should be calculated separately, not just use
displayPosition().

2) Fixes the the bug when loading 16-bit data. KoBgrU16Traits::alpha_pos
is *not* the offset of the alpha channel :) Now all the offset
calculations are done by the compiler in the templated code. At least
for RGB color spaces. We need to do the same for Lab, CMYK and Gray
spaces.