LoudnessBuffer for Meter, Graph, etc.

[exa04]

This buffer will need to compute the loudness of incoming audio in large overlapping blocks by pre-filtering each channel, taking their respective mean square values, summing them, gating the result, and averaging it. Making this work will involve adding (likely 2nd order IIR) filters to Cyma, either through an external crate, or a new internal filter struct.

This PDF explains the process in great detail.