Merge pull request #28615 from Kumataro:fix28606

imgcodecs(png): support cICP metadata for imreadWithMetadata() #28615

Close https://github.com/opencv/opencv/issues/28606
Related https://github.com/opencv/opencv/pull/27741 (support for imwriteWithMetadata)

### 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
- [x] 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_png.cpp

@@ -8,6 +8,13 @@ namespace opencv_test { namespace {
 
 #if defined(HAVE_PNG) || defined(HAVE_SPNG)
 
+// See https://github.com/opencv/opencv/pull/28615
+// Precision differences in 16-bit grayscale conversion between old and modern libpng versions
+#define OPENCV_IMGCODECS_PNG_EPS_DEFAULT (4)
+#ifndef OPENCV_IMGCODECS_PNG_EPS_16BIT_GRAY
+#define  OPENCV_IMGCODECS_PNG_EPS_16BIT_GRAY (OPENCV_IMGCODECS_PNG_EPS_DEFAULT)
+#endif
+
 TEST(Imgcodecs_Png, write_big)
 {
     const string root = cvtest::TS::ptr()->get_data_path();
@@ -276,10 +283,12 @@ TEST_P(Imgcodecs_Png_PngSuite, decode)
         cvtColor(gt_3, gt_258, COLOR_BGR2RGB);
     }
 
+    const double epsGrayAnydepth = ((gt.depth() == CV_16U) && (gt.channels() > 1)) ? OPENCV_IMGCODECS_PNG_EPS_16BIT_GRAY: OPENCV_IMGCODECS_PNG_EPS_DEFAULT;
+
     // Perform comparisons with different imread flags
     EXPECT_PRED_FORMAT2(cvtest::MatComparator(1, 0), imread(filename, IMREAD_GRAYSCALE), gt_0);
     EXPECT_PRED_FORMAT2(cvtest::MatComparator(1, 0), imread(filename, IMREAD_COLOR), gt_1);
-    EXPECT_PRED_FORMAT2(cvtest::MatComparator(4, 0), imread(filename, IMREAD_ANYDEPTH), gt_2);
+    EXPECT_PRED_FORMAT2(cvtest::MatComparator(epsGrayAnydepth, 0), imread(filename, IMREAD_ANYDEPTH), gt_2); // IMREAD_GRAYSCALE is used.
     EXPECT_PRED_FORMAT2(cvtest::MatComparator(0, 0), imread(filename, IMREAD_COLOR | IMREAD_ANYDEPTH), gt_3);
     EXPECT_PRED_FORMAT2(cvtest::MatComparator(1, 0), imread(filename, IMREAD_COLOR_RGB), gt_256);
     EXPECT_PRED_FORMAT2(cvtest::MatComparator(0, 0), imread(filename, IMREAD_COLOR_RGB | IMREAD_ANYDEPTH), gt_258);