FR : consider adding better than cubic interpolation

[geoffroymontel]

XPlayBuf could implement better than cubic interpolation.

• Voxengo offers a high quality resampler, an optimized sinc interpolation, and it can apparently used in realtime apps. But it seems only good for static resampling ... so maybe not a good idea
• libsamplerate offers sinc interpolation
• Justin Frankel / Cockos / Reaper's WDL offers sinc interpolation as well