Add the AVIF_STRICT_ALPHA_ISPE_REQUIRED flag

To allow alpha auxiliary image items to not have an ispe property, clear
the AVIF_STRICT_ALPHA_ISPE_REQUIRED bit in decoder->strictFlags.

Update the bug numbers in comments.

include/avif/avif.h

diff --git a/include/avif/avif.h b/include/avif/avif.h
index 124d173..e4f01c6 100644
--- a/include/avif/avif.h
+++ b/include/avif/avif.h
@@ -723,8 +723,16 @@
     // function returns AVIF_FALSE and this strict flag is set, the decode will fail.
     AVIF_STRICT_CLAP_VALID = (1 << 1),
 
+    // Requires the ImageSpatialExtentsProperty ('ispe') be present in alpha auxiliary image items.
+    // avif-serialize 0.7.3 or older does not add the 'ispe' item property to alpha auxiliary image
+    // items. If you need to decode AVIF images encoded by the cavif encoder with avif-serialize
+    // 0.7.3 or older, be sure to disable this bit. (This issue has been corrected in avif-serialize
+    // 0.7.4.) See https://github.com/kornelski/avif-serialize/issues/3 and
+    // https://crbug.com/1246678.
+    AVIF_STRICT_ALPHA_ISPE_REQUIRED = (1 << 2),
+
     // Maximum strictness; enables all bits above. This is avifDecoder's default.
-    AVIF_STRICT_ENABLED = AVIF_STRICT_PIXI_REQUIRED | AVIF_STRICT_CLAP_VALID
+    AVIF_STRICT_ENABLED = AVIF_STRICT_PIXI_REQUIRED | AVIF_STRICT_CLAP_VALID | AVIF_STRICT_ALPHA_ISPE_REQUIRED
 } avifStrictFlag;
 typedef uint32_t avifStrictFlags;
 

src/read.c

diff --git a/src/read.c b/src/read.c
index ad22f0b..3b38148 100644
--- a/src/read.c
+++ b/src/read.c
@@ -3101,9 +3101,19 @@
                 avifDiagnosticsPrintf(data->diag, "Item ID [%u] size is too large [%ux%u]", item->id, item->width, item->height);
                 return AVIF_RESULT_BMFF_PARSE_FAILED;
             }
-        } else if (!item->auxForID) { // NON-STANDARD: Allow auxiliary images to not have an ispe property. See: https://crbug.com/1245673
-            avifDiagnosticsPrintf(data->diag, "Item ID [%u] is missing a mandatory ispe property", item->id);
-            return AVIF_RESULT_BMFF_PARSE_FAILED;
+        } else {
+            const avifProperty * auxCProp = avifPropertyArrayFind(&item->properties, "auxC");
+            if (auxCProp && isAlphaURN(auxCProp->u.auxC.auxType)) {
+                if (decoder->strictFlags & AVIF_STRICT_ALPHA_ISPE_REQUIRED) {
+                    avifDiagnosticsPrintf(data->diag,
+                                          "[Strict] Alpha auxiliary image item ID [%u] is missing a mandatory ispe property",
+                                          item->id);
+                    return AVIF_RESULT_BMFF_PARSE_FAILED;
+                }
+            } else {
+                avifDiagnosticsPrintf(data->diag, "Item ID [%u] is missing a mandatory ispe property", item->id);
+                return AVIF_RESULT_BMFF_PARSE_FAILED;
+            }
         }
     }
     return avifDecoderReset(decoder);
@@ -3428,7 +3438,7 @@
         if (alphaItem) {
             if (!alphaItem->width && !alphaItem->height) {
                 // NON-STANDARD: Alpha subimage does not have an ispe property; adopt width/height from color item
-                // See: https://crbug.com/1245673
+                assert(!(decoder->strictFlags & AVIF_STRICT_ALPHA_ISPE_REQUIRED));
                 alphaItem->width = colorItem->width;
                 alphaItem->height = colorItem->height;
             }