Allow decoding of 64bit png images.

Fixes a part of ticket #639.

libavcodec/pngdec.c

@@ -485,6 +485,9 @@ static int decode_frame(AVCodecContext *avctx,
                 } else if (s->bit_depth == 16 &&
                            s->color_type == PNG_COLOR_TYPE_RGB) {
                     avctx->pix_fmt = PIX_FMT_RGB48BE;
+                } else if (s->bit_depth == 16 &&
+                           s->color_type == PNG_COLOR_TYPE_RGB_ALPHA) {
+                    avctx->pix_fmt = PIX_FMT_RGBA64BE;
                 } else if (s->color_type == PNG_COLOR_TYPE_PALETTE) {
                     avctx->pix_fmt = PIX_FMT_PAL8;
                 } else if (s->bit_depth == 1) {