Pass true to avifDecoderDataGenerateImageGridTiles

Pass alpha=AVIF_TRUE to the second
avifDecoderDataGenerateImageGridTiles() call, which operates on
data->alphaGrid.

src/read.c

diff --git a/src/read.c b/src/read.c
index 3def77e..33ddc46 100644
--- a/src/read.c
+++ b/src/read.c
@@ -2275,7 +2275,7 @@
         }
 
         if ((data->alphaGrid.rows > 0) && (data->alphaGrid.columns > 0) && alphaOBUItem) {
-            if (!avifDecoderDataGenerateImageGridTiles(data, &data->alphaGrid, alphaOBUItem, AVIF_FALSE)) {
+            if (!avifDecoderDataGenerateImageGridTiles(data, &data->alphaGrid, alphaOBUItem, AVIF_TRUE)) {
                 return AVIF_RESULT_INVALID_IMAGE_GRID;
             }
             data->alphaTileCount = data->tiles.count - data->colorTileCount;