Keepalive frames no longer leak uninitialized memory; NVENC AV1 added

Fixes

• Emit black keepalive frames from NativeVideoSource instead of uninitialized memory. webrtc::I420Buffer::Create leaves the pixel planes uninitialized, so the pre-capture keepalive frames could leak recycled heap contents (often fragments of earlier frames from the same process) to subscribers as the first keyframes - #1271 (@eh-steve)
• Add NVIDIA NVENC AV1 encoding when the GPU reports AV1 encode support.