Render performance optimization

[rualark]

Key performance hogs:

• Reverb and limiter slow down rendering of a stage with small number of instruments 3 times. Rendering time of stage with many instruments is not influenced by limiter or reverb.
• Rendering multiple stages is always a big problem.
• Instruments are not very fast

Optimization approaches:

• Replace reverb and limiter with faster
• Switch reverb and limiter off in all stages except stage 0
• Build single stage with multiple instruments