fix(wechat_code): Modify isnan for compatibility with -ffast_math.

[Konano]

fix #3150
Merge with opencv/opencv#23881

Reference: https://stackoverflow.com/questions/7263404/mingw32-stdisnan-with-ffast-math

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

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
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  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[opencv-alalek]

Lets reuse OpenCV implementation: https://github.com/opencv/opencv/blob/4.7.0/modules/core/include/opencv2/core/fast_math.hpp#L284

Use cvIsNaN() directly.

[asmorkalov]

@Konano Friendly reminder.

[Konano]

Suggested change

	inline bool isnan(float v) {
	union { float v; uint32_t x; } u = { v };
	return (u.x & 0x7fffffffu) > 0x7f800000u;
	}
	inline bool isnan(double v) {
	union { double v; uint64_t x; } u = { v };
	return (u.x & ~0x8000000000000000uLL) > 0x7ff0000000000000uLL;
	}
	inline bool isnan(float v) { return cvIsNaN(v); }
	inline bool isnan(double v) { return cvIsNaN(v); }

I called cvIsNaN() directly but it doesn't seem to work, it still crashed in the local test.

The new line #include "opencv2/core.hpp" is added in the import section.

[Konano]

Compile command:

cmake -G Ninja \
  -D CMAKE_BUILD_TYPE=DEBUG \
  -D CMAKE_INSTALL_PREFIX=../release \
  -D OPENCV_GENERATE_PKGCONFIG=YES \
  -D OPENCV_EXTRA_MODULES_PATH=../opencv_contrib-4.7.0/modules \
  -D BUILD_opencv_xfeatures2d=OFF \
  -D BUILD_opencv_freetype=ON \
  -D ENABLE_FAST_MATH=ON ..
ninja -j 24
ninja install
g++ test.cpp -o test -Wall -g `pkg-config --cflags opencv4 --libs opencv4`

Any wrong?

[Konano]

@asmorkalov @opencv-alalek Need help.

[asmorkalov]

👍