avifgainmaputil: fix incorrectly printed resolution (#3264)

apps/avifgainmaputil/imageio.cc

diff --git a/apps/avifgainmaputil/imageio.cc b/apps/avifgainmaputil/imageio.cc
index c127d06..05b55f6 100644
--- a/apps/avifgainmaputil/imageio.cc
+++ b/apps/avifgainmaputil/imageio.cc
@@ -157,7 +157,7 @@
 
   avifRWData encoded = AVIF_DATA_EMPTY;
   std::cout << "AVIF to be written:\n";
-  avifImageDump(image, grid_cols, grid_rows,
+  avifImageDump(grid_cells_ptrs[0], grid_cols, grid_rows,
                 AVIF_PROGRESSIVE_STATE_UNAVAILABLE);
   PrintEncodingSettings(encoder, image->gainMap != nullptr);
   avifResult result = avifEncoderAddImageGrid(encoder, grid_cols, grid_rows,