avcodec/pixlet: fix architecture-dependent code and values

The constants used in the decoder used floating point precision,
and this caused different values to be generated on different
architectures.

So, eradicate floating point numbers and use fixed point (32.32)
arithmetics everywhere, replacing constants with precomputed integer
values.

Signed-off-by: Vittorio Giovara <vittorio.giovara at gmail.com>
Signed-off-by: Paul B Mahol <onemda@gmail.com>