aom encoder: improve performance by ~2x using new 'all intra' mode

Available from aom v3.1.0, this usage mode is considered the
default setting for still images.

It uses a single pass and removes all keyframe logic, resulting
in significantly fewer memory allocations and halves CPU time.

libavif has already adopted this as the default.

libheif/heif_encoder_aom.cc

@@ -689,7 +689,10 @@ struct heif_error aom_encode_image(void* encoder_raw, const struct heif_image* i
 
 
   unsigned int aomUsage = 0;
-#if defined(AOM_USAGE_REALTIME)
+#if defined(AOM_USAGE_ALL_INTRA)
+  // aom 3.1.0
+  aomUsage = (encoder->realtime_mode ? AOM_USAGE_REALTIME : AOM_USAGE_ALL_INTRA);
+#elif defined(AOM_USAGE_REALTIME)
   // aom 2.0
   aomUsage = (encoder->realtime_mode ? AOM_USAGE_REALTIME : AOM_USAGE_GOOD_QUALITY);
 #endif