Use src_set_ratio to fix artifacts in Sample playback

[sakertooth]

Makes use of the function src_set_ratio to allow for smoother transitions in the resampling ratio when playing back a Sample, removing the artifacts heard when de-tuning notes for instruments like the AFP down rapidly.

[DomClark]

According to the documentation, the point of src_set_ratio is to avoid a smooth transition, not to create one. However, if this fixes artefacts in playback, then it's fine by me.

[sakertooth]

According to the documentation, the point of src_set_ratio is to avoid a smooth transition, not to create one. However, if this fixes artefacts in playback, then it's fine by me.

Ah, I must've misread the documentation. It does fix the artifacts from my testing though, so I guess a step-like change in the ratio is what was needed. Ideally, we should move away from using libsamplerate in Sample playback altogether in the future though. Using the functions in interpolation.h instead provides simpler code and avoids us having do a heap allocation to feed into libsamplerate, and of course helps to avoid problems like these.