Improve lbx_colour structure layout.

Improve lbx_colour structure layout.

There is no point in using bit-fields for the components of this
structure.  The structure will never be smaller than 4 bytes so
there is no space savings compared to just using four unsigned
char members.  This avoids the compiler needing to do any kind
of bit shuffling.

In the typical case where such a structure has no padding, this
means the layout of these members will now exactly match the PAM
format, enabling a modest speedup.