feat: encode VP9 in hardware on Windows, when turned on

[devopvoid]

Summary

Stacked on #347 (hardware H.265 on Windows), and based on its branch until that is merged.

Intel GPUs have a Media Foundation VP9 encoder. The hardware encoder factory now uses it for VP9 (profile 0), the way it uses the H.264, H.265 and AV1 encoders — but only when it is turned on:

System.setProperty("webrtc.hardware.vp9Encoder", "true"); // before the factory is created

It is off by default because it has not been tested on an Intel GPU, and once on it takes over VP9 from libvpx for every stream.

• The property is read natively (HardwareCodecOptions, through Boolean.getBoolean) when the Windows hardware factories are made, so it reaches both the codec list and the encoders of a call, with no new API. The read is safe from any thread: its own local frame, nothing done while another exception is pending.
• Without the property, or where there is no VP9 encoder, VP9 stays with libvpx. A hardware encoder that fails to start, or a stream that asks for SVC, falls back to libvpx too, through the existing fallback chain. VP9 is offered by the software encoders anyway, so turning it on changes nothing that is negotiated.
• EncoderOutputProcessor passes VP9 frames on as they come, with the VP9 information of a single layer that the Java encoders get as well (the group of frames with every key frame), parses the QP and uses the VP9 quality scaling thresholds of the Java encoders.
• The factory Javadoc and the codec guide describe the property.

Testing

On Windows x64 (AMD Radeon RX 9070 XT):

• vp9HardwareEncodingIsOptIn: without the property, a VP9 call is encoded by libvpx. It runs on every Windows and Linux machine, CI included.
• hardwareEncodesVp9WhenEnabled: with the property, a VP9 call is encoded in hardware where there is a VP9 encoder; webrtc.test.hardwareVp9Encoder makes it fail rather than skip without one. This machine has an Intel VP9 encoder registered but no Intel GPU: the factory found it, the encoder failed to start (E_FAIL) and libvpx took over, so the call went on and the test skipped.
• The full webrtc test suite: 277 tests, no failures.

Not verified

VP9 encoding on an actual Intel GPU: neither this machine nor CI has one. That is why it is opt-in.