Sample-exact models: improve
- Remove the redundant hasSampleExactData() function. Instead, signal lack of s.ex.data by returning a NULL in valueBuffer()
- Cache s.ex.buffers and only update them once per period
- Make valueBuffer() in AutomatableModel threadsafe so that it can be used for NPH's sharing the same model
- Add sample-exactness to instrumenttrack's vol & pan knobs