It's allocated and freed from Open/Close now.

For now, only Marvell decoders seem to assert if channels count is 0.

And don't do a late opening if there is no decoder available.

Disabled by default. Enable it with the "mediacodec-audio" option.

Add ParseVideoExtra that parse video extra data for h264/hevc.
Leave room for a possible ParseAudioExtra.

Rename DecodeVideo to DecodeCommon.

DecodeCommon use 2 callbacks to put Video and Audio specific code. These
callbacks are called when a new block is processed and when it try to get an
output buffer.

Add an union for future audio arguments.

It happens when fetching the "channel-mask" property.

Disabled since it's not tested enough.

Also, fix it.