libavcodec/zmbv: change 24-bit decoder channel order, from RGB24 to BGR24

libavcodec/zmbv: change 24-bit decoder channel order, from RGB24 to BGR24

This brings the channel order in line with that used in 32-bit mode (BGR0).

24-bit decoding is disabled by default (#ifdef ZMBV_ENABLE_24BPP), and no
prior encoders or sample videos are known to exist for this bit depth, so
I consider this change in implementation is unlikely to affect anyone.

The decision has been made in agreement with the DOSBox Development Team
(dosbox.crew@gmail.com), specifically with harekiet, who wrote the original
codec.