Support 48bpp pam decoding.

Fixes ticket #882.

libavcodec/pnm.c

@@ -122,9 +122,7 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
             if (maxval < 256) {
             avctx->pix_fmt = PIX_FMT_RGB24;
             } else {
-                av_log(avctx, AV_LOG_ERROR, "16-bit components are only supported for grayscale\n");
-                avctx->pix_fmt = PIX_FMT_NONE;
-                return -1;
+                avctx->pix_fmt = PIX_FMT_RGB48BE;
             }
         } else if (depth == 4) {
             avctx->pix_fmt = PIX_FMT_RGB32;