avcodec/j2kenc: Replace RGB24 special case by generic test

This fixes RGB48 with libavcodec as decoder

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ad4d647591)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/j2kenc.c

@@ -1530,6 +1530,7 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     int tileno, ret;
     Jpeg2000EncoderContext *s = avctx->priv_data;
     uint8_t *chunkstart, *jp2cstart, *jp2hstart;
+    const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(avctx->pix_fmt);
 
     if ((ret = ff_alloc_packet(avctx, pkt, avctx->width*avctx->height*9 + AV_INPUT_BUFFER_MIN_SIZE)) < 0)
         return ret;
@@ -1586,7 +1587,7 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
         bytestream_put_byte(&s->buf, 1);
         bytestream_put_byte(&s->buf, 0);
         bytestream_put_byte(&s->buf, 0);
-        if (avctx->pix_fmt == AV_PIX_FMT_RGB24 || avctx->pix_fmt == AV_PIX_FMT_PAL8) {
+        if ((desc->flags & AV_PIX_FMT_FLAG_RGB) || avctx->pix_fmt == AV_PIX_FMT_PAL8) {
             bytestream_put_be32(&s->buf, 16);
         } else if (s->ncomponents == 1) {
             bytestream_put_be32(&s->buf, 17);