class MixerSource

Class that manages relevant information of a `MixerStage` source to mix onto destination stream.

This consists of the computation of the combined gain to be applied into the source stream, as

well as processing the samples in the source stream to mix onto the destination stream using an

appropriate sampler implementation with respect to the synchronization needs between the source

and destination streams.

Combined gain computation is done by using the set of `GainControl`s that are connected to the

source, combined with the `GainControl`s that are connected to the destination. The source gain

controls are selected during the creation of this mixer source edge, and are accessed by the

`options.gain_ids`. Similarly, the initial destination gain controls are passed via

`dest_gain_ids` at construction to build the total set of gain controls to be combined. However,

while the source gain controls are guaranteed to remain constant after the edge creation, the

destination gain controls can be modified after the creation of this mixer source. Therefore, we

keep track of the destination gain controls and update them via `SetDestGains` as requested.

Once the combined gain is computed and updated via `PrepareSourceGainForNextMix` call,

corresponding source samples can be processed and mixed onto the destination stream via `Advance`

and `Mix` calls. These mainly follow the same `PipelineStage::Advance` and `PipelineStage::Read`

call patterns, where `Read` roughly expands to a sequence of `PrepareSourceGainForNextMix` and

`Mix` calls respectively.