ptx: correct decoding

The image data is in BGR and not in RGB.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/ptx.c

@@ -58,7 +58,7 @@ static int ptx_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
         return -1;
     }
 
-    avctx->pix_fmt = PIX_FMT_RGB555LE;
+    avctx->pix_fmt = PIX_FMT_BGR555LE;
 
     if (buf_end - buf < offset)
         return AVERROR_INVALIDDATA;

tests/ref/fate/ptx

 #tb 0: 1/25
-0,          0,          0,        1,   393216, 0x019d9950
+0,          0,          0,        1,   393216, 0x30479950