fix: resolve crashes caused by using the ffmpeg library

[julee]

• Addressed issue where repeated reloading of the ffmpeg library led to crashes. Previously, each component attempted to load the ffmpeg library and executed an unload before loading, causing other components to retain stale function addresses if the library was reloaded at a different address. This issue was particularly prevalent on macOS.
• Fixed undefined behavior due to incorrect size assumption of std::function objects. On certain platforms like arm64 macOS, sizeof(std::function<void()>) is 32, not the same as a pointer size. This discrepancy led to undefined behavior when forcibly casting.

This fix ensures that the library is loaded only once per process and corrects the handling of std::function objects to prevent crashes.

[ChristianFeldmann]

Hi! Thanks for the report. I have not heard of this undefined behavior so far. Do you have a link to this? Maybe an article or a bugreport?
In the next change, I will replace the ffmpeg integration with https://github.com/ChristianFeldmann/LibFFmpeg
. That is essentially the code from here extracted into a separate tested library. In that other repository I have added many tests on many different platforms including M1 and I have not seen any issues with is there.

[julee]

The core reason for the crash is that after unloading the dynamic library, the original symbol addresses are still being used. This is not undefined behavior, but a clear bug!

consider code:

//compoent A:
QLibrary lib;
lib.setFileName(libAVCodecPath);
lib.unload(); //call by unloadAllLibraries
lib.load();
void *fucntionPtr1 =  lib.resolve(symbolNameSuchAsAVCodecOpen);

//compoent B:
QLibrary lib;
lib.setFileName(libAVCodecPath);
// as shared library loading is global for one process, so here, lib.unload() will cause libAVCodecPath loaded in compoent A unload!
lib.unload(); //call by unloadAllLibraries
lib.load();
void *fucntionPtr2 =  lib.resolve(symbolNameSuchAsAVCodecOpen); // this address may be different with fucntionPtr1

application go on run:
compoentA.runXXXUse_fucntionPtr1();   // go on use: functionPtr1, but this address is invalid,

[julee]

Can you continue to evaluate this issue?