lavc/vmnc: Fix 32bit colour-space, there is no transparency.

libavcodec/vmnc.c

@@ -538,7 +538,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
         avctx->pix_fmt = AV_PIX_FMT_RGB555;
         break;
     case 32:
-        avctx->pix_fmt = AV_PIX_FMT_RGB32;
+        avctx->pix_fmt = AV_PIX_FMT_0RGB32;
         break;
     default:
         av_log(avctx, AV_LOG_ERROR, "Unsupported bitdepth %i\n", c->bpp);