Merge pull request #26849 from sturkmen72:apng-writeanimation

APNG encoding optimization #26849

related #26840

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/imgcodecs/test/test_common.cpp

@@ -48,4 +48,21 @@ Mat generateTestImageGrayscale()
     return image;
 }
 
+void readFileBytes(const std::string& fname, std::vector<unsigned char>& buf)
+{
+    FILE * wfile = fopen(fname.c_str(), "rb");
+    if (wfile != NULL)
+    {
+        fseek(wfile, 0, SEEK_END);
+        size_t wfile_size = ftell(wfile);
+        fseek(wfile, 0, SEEK_SET);
+
+        buf.resize(wfile_size);
+        size_t data_size = fread(&buf[0], 1, wfile_size, wfile);
+        fclose(wfile);
+
+        EXPECT_EQ(data_size, wfile_size);
+    }
+}
+
 }  // namespace