Support gray8a pam decoding.

libavcodec/pnm.c

@@ -118,6 +118,9 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
             } else {
                 avctx->pix_fmt = PIX_FMT_GRAY16BE;
             }
+        } else if (depth == 2) {
+            if (maxval == 255)
+                avctx->pix_fmt = PIX_FMT_GRAY8A;
         } else if (depth == 3) {
             if (maxval < 256) {
             avctx->pix_fmt = PIX_FMT_RGB24;

libavcodec/pnmdec.c

@@ -75,6 +75,11 @@ static int pnm_decode_frame(AVCodecContext *avctx, void *data,
         if (s->maxval < 255)
             upgrade = 1;
         goto do_read;
+    case PIX_FMT_GRAY8A:
+        n = avctx->width * 2;
+        components=2;
+        sample_len=8;
+        goto do_read;
     case PIX_FMT_GRAY16BE:
     case PIX_FMT_GRAY16LE:
         n = avctx->width * 2;