Set a callback function for dav1d_log()

The callback function directs dav1d_log() output to the error message
buffer in the avifDiagnostics struct.

src/codec_dav1d.c

diff --git a/src/codec_dav1d.c b/src/codec_dav1d.c
index 5aff5a1..666dcef 100644
--- a/src/codec_dav1d.c
+++ b/src/codec_dav1d.c
@@ -15,6 +15,7 @@
 #pragma clang diagnostic pop
 #endif
 
+#include <stdio.h>
 #include <string.h>
 
 // For those building with an older version of dav1d (not recommended).
@@ -37,6 +38,12 @@
     (void)cookie;
 }
 
+static void avifDav1dLogCallback(void * cookie, const char * format, va_list ap)
+{
+    avifCodec * codec = (avifCodec *)cookie;
+    vsnprintf(codec->diag->error, AVIF_DIAGNOSTICS_ERROR_BUFFER_SIZE, format, ap);
+}
+
 static void dav1dCodecDestroyInternal(avifCodec * codec)
 {
     if (codec->internal->hasPicture) {
@@ -70,6 +77,8 @@
         // a message, so we set frame_size_limit to at most 8192 * 8192 to avoid the dav1d_log
         // message.
         dav1dSettings.frame_size_limit = (sizeof(size_t) < 8) ? AVIF_MIN(codec->imageSizeLimit, 8192 * 8192) : codec->imageSizeLimit;
+        dav1dSettings.logger.cookie = codec;
+        dav1dSettings.logger.callback = avifDav1dLogCallback;
         dav1dSettings.operating_point = codec->operatingPoint;
         dav1dSettings.all_layers = codec->allLayers;