OSS-Fuzz: add generic buffer saver

Introduce a generic buffer saver that attempts to save images in the
format matching the input. This ensures that our fuzzers covers the
following savers: `heifsave`, `ppmsave`, `radsave` and `tiffsave`.

fuzz/generic_buffer_fuzzer.cc

@@ -0,0 +1,64 @@
+#include <vips/vips.h>
+
+extern "C" int
+LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	vips_concurrency_set(1);
+	return 0;
+}
+
+extern "C" int
+LLVMFuzzerTestOneInput(const guint8 *data, size_t size)
+{
+	VipsImage *image;
+	void *buf;
+	const char *loader, *loader_end;
+	char *tmp, *suffix;
+	size_t len;
+
+	if (!(image = vips_image_new_from_buffer(data, size, "", nullptr)))
+		return 0;
+
+	if (image->Xsize > 100 ||
+		image->Ysize > 100 ||
+		image->Bands > 4) {
+		g_object_unref(image);
+		return 0;
+	}
+
+	if (vips_image_get_typeof(image, VIPS_META_LOADER) &&
+		vips_image_get_string(image, VIPS_META_LOADER, &loader)) {
+		g_object_unref(image);
+		return 0;
+	}
+
+	loader_end = g_strrstr(loader, "load");
+	g_assert(loader_end);
+
+	tmp = g_strndup(loader, strlen(loader) - strlen(loader_end));
+	suffix = g_strconcat(".", tmp, NULL);
+	g_free(tmp);
+
+	if (strcmp(suffix, ".rad") == 0) {
+		g_free(suffix);
+		// .rad -> .hdr
+		suffix = g_strdup(".hdr");
+	}
+	else if (strcmp(suffix, ".heif") == 0) {
+		g_free(suffix);
+		// Set the default compression for heifsave to AV1 instead of HEVC
+		suffix = g_strdup(".avif");
+	}
+
+	if (vips_image_write_to_buffer(image, suffix, &buf, &len, nullptr)) {
+		g_free(suffix);
+		g_object_unref(image);
+		return 0;
+	}
+
+	g_free(buf);
+	g_free(suffix);
+	g_object_unref(image);
+
+	return 0;
+}

fuzz/meson.build

@@ -20,6 +20,7 @@ fuzz_progs = [
     'pngsave_buffer_fuzzer',
     'webpsave_buffer_fuzzer',
     'gifsave_buffer_fuzzer',
+    'generic_buffer_fuzzer',
     'sharpen_fuzzer',
     'thumbnail_fuzzer',
     'smartcrop_fuzzer',