Draft: Add option for offline processing

[vackva]

For offline processing scenarios (such as rendering in a DAW), there is no guaranteed processing time per block. Instead, the processBlock callback may be invoked immediately after the previous call returns. Since anira requires the full processing time per block to avoid missing samples, we need to introduce a blocking mechanism when non-real-time mode is enabled.

@faressc, please let me know your thoughts on this approach.

I have an implementation that demonstrates this approach works, but I still need to update our examples and documentation.