Improvement of FxMixer multithreading
Use dynamic building of jobqueues with dependency counting:
- At the start, each channel that has no dependencies is added automatically to the queue
- Then, after each channel is processed, it increments the dep.counter of all its recipients
- When a channel's dep.counter hits the amount of its dependencies (senders), it gets automatically added to the queue
- The queue is finished when the master channel has been processed
- Muted channels are automatically processed at the start regardless dependencies, because they don't have to care about senders, being muted

Hopefully this will improve Fx Mixer performance.
Fix segfault when enabling/disabling sends while playing
More updates to FxMixer - better handling of muted channels
Merge pull request #1305 from diizy/stable-1.1

Improvement of FxMixer multithreading