Previously this was restricted to 2 input buses and 2 output buses, but
this has been lifted to 254. When the number exceeds 2, internal data
structures will be allocated on the heap, otherwise they'll use a local
array contained within the ma_node structure.

This commit changes the node configuration. Previously there was a
fixed sized array for specifying the channel counts for each bus. This
array must now be defined outside of the config by the caller. The
following config variables have been renamed:

  * inputChannels > pInputChannels
  * outputChannels > pOutputChannels

This commit also adds the ability to configure input and output bus
counts on a per-instance basis rather than via the node vtable. To do
this, set the bus count in the vtable to MA_NODE_BUS_COUNT_UNKNOWN.
This will tell miniaudio to look at the node config to determine the
bus count rather than the vtable. It's an error to specify this in the
node config if the vtable specifies anything other than
MA_NODE_BUS_COUNT_UNKNOWN.