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.

libavcodec/zmbv.c

@@ -473,7 +473,7 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *got_frame, AVPac
             c->bpp = 24;
             decode_intra = zmbv_decode_intra;
             c->decode_xor = zmbv_decode_xor_24;
-            avctx->pix_fmt = AV_PIX_FMT_RGB24;
+            avctx->pix_fmt = AV_PIX_FMT_BGR24;
             c->stride = c->width * 3;
             break;
 #endif //ZMBV_ENABLE_24BPP