Support camera-specific hwaccel_args for Timelapse export

[aroberts]

Describe what you are trying to accomplish and why in non technical terms
Timelapse export hardware acceleration is driven by the global ffmpeg.hwaccel_args config, as well as several other things. I have one camera that is not able to export using these settings, because the resolution is greater than the max allowed by my igpu under vaapi. I'd like to be able to override the global ffmpeg hwaccel args for that camera (to empty, in my case, but it should allow any args) for Timelapse export, so I don't have to globally disable hardware acceleration

Describe the solution you'd like
add optional camera config key record.export.hwaccel_args (alongside timelapse_args), with a default value of "auto" which would mean the value stored in [camera].hwaccel_args (which in turn defaults to the global hwaccel_args). From there, the export ffmpeg construction line here can instead look at the camera-specific config, as it does for timelapse_args.

Describe alternatives you've considered
Removing the global hwaccel_args and instead assigning it per-camera appears to have consequences for other features (Birdseye, for example, appears to also use the global setting with no way to override), as well as being more config to manage.