avcodec/xwdenc: do not rely on AV_PIX_FMT_FLAG_PSEUDOPAL palettes

This is the only code I found within FFmpeg that still inherently
requires AV_PIX_FMT_FLAG_PSEUDOPAL. It's easily changed not to require
it.

Preparation for the next patch.

libavcodec/xwdenc.c

@@ -41,6 +41,7 @@ static int xwd_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     int i, out_size, ret;
     uint8_t *ptr, *buf;
     AVFrame * const p = (AVFrame *)pict;
+    uint32_t pal[256];
 
     pixdepth = av_get_bits_per_pixel(desc);
     if (desc->flags & AV_PIX_FMT_FLAG_BE)
@@ -180,11 +181,17 @@ static int xwd_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
     bytestream_put_be32(&buf, 0);             // window border width
     bytestream_put_buffer(&buf, WINDOW_NAME, WINDOW_NAME_SIZE);
 
+    if (pix_fmt == AV_PIX_FMT_PAL8) {
+        memcpy(pal, p->data[1], sizeof(pal));
+    } else {
+        avpriv_set_systematic_pal2(pal, pix_fmt);
+    }
+
     for (i = 0; i < ncolors; i++) {
         uint32_t val;
         uint8_t red, green, blue;
 
-        val   = AV_RN32A(p->data[1] + i * 4);
+        val   = pal[i];
         red   = (val >> 16) & 0xFF;
         green = (val >>  8) & 0xFF;
         blue  =  val        & 0xFF;