gltfpack: Use ERR Base64 embedded data for invalid images

This prevents errors around missing images in case we can't find the source image
or can't encode it. Loading the resulting glTF file will result in a PNG decode
error.

Unfortunately, files like this are still treated as invalid by glTF spec/validator.

gltf/write.cpp

@@ -948,7 +948,9 @@ void writeImage(std::string& json, std::vector<BufferView>& views, const cgltf_i
 
 void writeImageError(std::string& json, const char* action, size_t index, const char* uri, const char* reason)
 {
-	(void)json;
+	append(json, "\"uri\":\"");
+	append(json, "data:image/png;base64,ERR/");
+	append(json, "\"");
 
 	fprintf(stderr, "Warning: unable to %s image %d (%s), skipping%s%s%s\n", action, int(index), uri ? uri : "embedded", reason ? " (" : "", reason ? reason : "", reason ? ")" : "");
 }