The function will be used in pa_sink_input_new() and
pa_source_output_new(). The fallback parameters are used to merge the
data in the format info with the sink/source sample spec and channel
map, when the format info is lacking some information.
This also fixes an issue in pa_format_info_to_sample_spec(): it did
no validation for the channels value. Now the validation is taken care
of in pa_format_info_get_channels().
This also fixes an issue in pa_format_info_to_sample_spec(): it did
no validation for the rate value. Now the validation is taken care of
in pa_format_info_get_rate().
I will need to use the function from outside libpulse.
I added the channel map argument, because the function will be called
from another function that is expected to initialize the channel map.
I don't know if it's in practice necessary, but it shouldn't do any
harm either.
