feat: add hardware frame context wrappers

[jb-alvarado]

Expose safe wrappers for FFmpeg hardware device and frame contexts, including frame allocation and software-to-hardware uploads. Allow video encoders to receive a cloned hardware frame context before opening.

[AdrianEddy]

this needs the full av_frame_copy_props, not only pts

[AdrianEddy]

1. No way to wrap an existing device or frames context. Device can only be built by av_hwdevice_ctx_create, and Frames only by allocating a fresh one. So the decode-hw → encode-hw zero-copy path is impossible with this API: you can't take a decoder's (*frame).hw_frames_ctx and hand it to the encoder.
2. I'd be nice to expose the initial_pool_size, as some backends need to tune that value
3. pix_fmt isn't set, and the doc comment doesn't say you must. avcodec_open2 fails if pix_fmt isn't the hardware format. Since Frames already knows its format, set_hardware_frames could just do it.
4. I'd be nice to add download() too for symmetry
5. No set_hardware_device on the encoder.
6. Device::create takes the raw AVHWDeviceType ffi enum, which leaks ffi into a safe wrapper.

[jb-alvarado]

Thanks @AdrianEddy, for reviewing my pull request. I implemented your suggestions and added also an example.