Wrong FPS value returned from some mp4 files

#### **Describe the bug**
Currently the FPS value is calculated using the stts box.
https://github.com/Exiv2/exiv2/blob/d17cfb6c7534d061d083500c60f4abe6c93ce277/src/quicktimevideo.cpp#L1172-L1173

And timeScale_ is set in movieHeaderDecoder, reading from the mvhd box.
https://github.com/Exiv2/exiv2/blob/d17cfb6c7534d061d083500c60f4abe6c93ce277/src/quicktimevideo.cpp#L1555-L1561

However the time scale value from mvhd is not supposed to be used here. Instead, each track contains a mdia box that defines a time scale value specific for this media in the mdhd box. The stts box belonging to this media should use the mdhd value and not the one from mvhd box.

<img width="800" alt="Image" src="https://github.com/user-attachments/assets/2db5700e-82e4-4fbb-90dc-2f1d7af653b8" />

<img width="800" alt="Image" src="https://github.com/user-attachments/assets/b5ed29bb-5132-4e44-a951-e7aa7f1545e9" />

Currently some mp4 files may work if the two values are identical, but not all files are so.


	break;
	case TimeScale:
	xmpData_["Xmp.video.TimeScale"] = buf.read_uint32(0, bigEndian);
	timeScale_ = buf.read_uint32(0, bigEndian);
	if (timeScale_ <= 0)
	timeScale_ = 1;
	break;

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Wrong FPS value returned from some mp4 files #3436

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	xmpData_["Xmp.video.FrameRate"] =
	static_cast<double>(totalframes) * static_cast<double>(timeScale_) / static_cast<double>(timeOfFrames);

Wrong FPS value returned from some mp4 files #3436

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