Support 16bpp grayscale pam decoding.

Fixes ticket #881.

libavcodec/pnm.c

@@ -111,10 +111,13 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
         avctx->height = h;
         s->maxval     = maxval;
         if (depth == 1) {
-            if (maxval == 1)
+            if (maxval == 1) {
                 avctx->pix_fmt = PIX_FMT_MONOWHITE;
-            else
+           } else if (maxval == 255) {
                 avctx->pix_fmt = PIX_FMT_GRAY8;
+           } else {
+                avctx->pix_fmt = PIX_FMT_GRAY16BE;
+            }
         } else if (depth == 3) {
             if (maxval < 256) {
             avctx->pix_fmt = PIX_FMT_RGB24;