obs-ffmpeg: Set AMF codec level properly

The default "level" setting was being used for each codec (AVC, HEVC,
AV1) supported by AMF. For example, all HEVC encoders were using
level 6.2 and this caused some playback devices to reject the
bitstream for decode because the device reported a maximum decode
level lower than 6.2.

Add functionality to determine the best match for the codec level
instead of relying on the defaults.