Add fuzz stuff ("backend/tests/fuzz"), including OSS-Fuzz

"Dockerfile" etc
PDF417: lessen some debug verbosity
gif: use "gif_" prefix; some code fiddling

backend/tests/fuzz/Dockerfile

@@ -0,0 +1,21 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+FROM gcr.io/oss-fuzz-base/base-builder
+RUN apt-get update && apt-get install -y make cmake
+RUN git clone --depth 1 https://git.code.sf.net/p/zint/code zint
+WORKDIR zint
+COPY build.sh $SRC/