gstalsa: Adding the possibility to not use alsa channel mapping
requested to merge hjespersen/gst-plugins-base:alsa_channel_mapping_should_be_optional_for_non_channel_layouts into master
It should be optional whether to use the channel-mapping or not, hence a property for enabling/disabling this. Default behavior is the same as previous. Disabling the channel-mapping will use the channel-layout as is.