Before this patch is applied we may never send the second message
because there's not room for it in the buffer and we can't return
failure-to-send (always interpreted as "retry") as we're in a void function.
Further, if you are on a mavlink2 connection we will not send out the
RC_CHANNELS_RAW message, depriving the user of any RC_CHANNELS messages.
This patch does have the drawback of doing more work on a mavlink1
connection - it has to fetch the data twice. On the other hand, it also
allows the GCS to set the message rates independently for both
RC_CHANNELS and RC_CHANNELS_RAW so one or the other can be squelched.
That could be handy for reducing bandwidth if you know you're not using
more than 8 input channels.
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