Rename speakers to channels, and remove an old incorrect comment
Pas the output device channel count to ALeffectState::process
Fallback to 16-bit samples in alffmpeg if a float32 format isn't supported
Move a declaration
Rename a couple parameters
Go to the next voice when a source is stopped
Only enable the BS2B filter with stereo output
Add a workaround for compilers without __COUNTER__

This can make GCC pretty noisey, complaining "declaration does not declare
anything" for each static_assert, but it should still function on such older
compilers.
Remove the long-deprecated 'format' config option
Allow selecting the 5.1-rear channel config from the config file
Reorder some options in the config sample
Remove the unused wide-stereo option