sgi: decode images with 4 channels at 8 and 16 bits

Changelog

@@ -11,6 +11,7 @@ version <next>:
 - XBM decoder
 - bmp standalone parser
 - OpenEXR image decoder
+- support decoding 4-channel SGI images
 
 
 version 10:

libavcodec/sgidec.c

@@ -198,8 +198,8 @@ static int decode_frame(AVCodecContext *avctx,
         avctx->pix_fmt = s->bytes_per_channel == 2 ? AV_PIX_FMT_GRAY16BE : AV_PIX_FMT_GRAY8;
     } else if (s->depth == SGI_RGB) {
         avctx->pix_fmt = s->bytes_per_channel == 2 ? AV_PIX_FMT_RGB48BE : AV_PIX_FMT_RGB24;
-    } else if (s->depth == SGI_RGBA && s->bytes_per_channel == 1) {
-        avctx->pix_fmt = AV_PIX_FMT_RGBA;
+    } else if (s->depth == SGI_RGBA) {
+        avctx->pix_fmt = s->bytes_per_channel == 2 ? AV_PIX_FMT_RGBA64BE : AV_PIX_FMT_RGBA;
     } else {
         av_log(avctx, AV_LOG_ERROR, "wrong picture format\n");
         return -1;