kgv1dec: Simplify kega decoding by using memcpy instead of loops

Fixes decoding errors with icc 13.1

Signed-off-by: Carl Eugen Hoyos <cehoyos@ag.or.at>
Use uint8_t instead of uint16_t pointer in kega decoder.

This change allows to remove a few casts and avoids
a potential pointer aliasing violation.
Make 32bit zmbv colour-space opaque.