Expose hardware devices to filter graphs

[happyme531]

Summary

• add a public HWDevice wrapper around an FFmpeg AVHWDeviceContext
• allow Graph(hw_device=device) and attach an independent AVBufferRef to filters declaring AVFILTER_FLAG_HWDEVICE before initialization
• keep a strong Python reference for the graph lifetime without exposing raw pointers
• document and type the API, including VideoFrame as a valid buffer template

Motivation

Hardware filters such as hwupload require AVFilterContext.hw_device_ctx, but PyAV currently has no public way to provide one. This implements the device-context part of the hardware-filter plumbing discussed in #2218. AVHWFramesContext plumbing for hardware-decoded buffer sources remains separate work.

Lifetime handling

HWDevice owns the reference returned by av_hwdevice_ctx_create. Each compatible filter receives its own av_buffer_ref before avfilter_init_dict, and normal filter-context destruction releases that reference. The graph also retains the Python HWDevice object.

Validation

• ruff format --check av tests
• isort --check-only av tests
• mypy av tests: success, 96 source files
• ARM64 wheel built on RK3588 against FFmpeg 8.1
• tests/test_filters.py: 18 passed
• downstream RKMPP validation covered software-frame hwupload, hardware scaling, hwdownload, and direct DRM PRIME encoding