Add support for custom user-managed data using AVIOContextOpaque

[wgxh-cc]

For asynchronous and integrated cases, the usage of AVIOContext could be awkward. Imagine that you have a stream data to sync across thread, which could be read and written continuously, then to link it to AVIOContext, we must work hard. With newly-added AVIOContextOpaque, we could do that:

let data: Arc<Mutex<Vec<u8>>> = Default::default();
let data_ = Arc::clone(&data);
thread::spawn(move || {
    data_.lock().unwrap().push(some_data)
});

let read_packet = |opaque, buf, len| {
    // just for demonstration here
    data.lock().unwrap().read_exact(buf).unwrap();

    len
};
let buffer = AVMem::new(1024);
let avio_ctx = AVIOContextOpaque::alloc_context(buffer, false, Arc::clone(&data), Some(read_packet), None, None);

[wgxh-cc]

Could I create a application-like example, demonstrating the API usage of rsmpeg and just enjoying?

If can, I shall put them under examples, so that one can run the pieces.

[Copilot]

Pull Request Overview

This PR adds support for custom user-managed data using AVIOContextOpaque, enabling more flexible integration with asynchronous and multi-threaded scenarios where custom data types can be passed through the AVIO context. The implementation allows users to provide their own data structures (wrapped in Arc<Mutex> or similar) alongside custom read/write/seek callbacks.

Key changes:

• Introduces AVIOContextOpaque struct with generic type support for custom data
• Refactors existing callback infrastructure to support generic opaque data types
• Updates AVIOContextContainer enum to include the new opaque variant

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/avformat/avio.rs	Adds AVIOContextOpaque implementation with generic type support and refactors callback infrastructure
src/avformat/avformat.rs	Updates AVIOContextContainer enum to include opaque variant and simplifies pointer access pattern

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[Copilot]

There are two write_c functions defined with conflicting configurations. The second function at line 124-132 should be pub to match the visibility of other functions in the module.

Suggested change

	unsafe extern "C" fn write_c<T: Send + Sync>(
	pub unsafe extern "C" fn write_c<T: Send + Sync>(

[Copilot]

The Drop implementation for AVIOContextOpaque doesn't handle the opaque data cleanup. The opaque pointer passed to avio_alloc_context needs to be properly freed, but there's no mechanism to recover and drop the Box<Opaque> that was leaked in alloc_context.

[Copilot]

Remove commented-out code. These type aliases are now defined in the opaque module and should not be left as dead comments.

Suggested change

// pub type SeekCallback = Box<dyn FnMut(&mut Vec<u8>, i64, i32) -> i64 + Send + 'static>;

[Copilot]

PhantomData is imported but never used in the code. Remove this unused import.

Suggested change

marker::PhantomData,