[osx] Increase CAEngine buffer size (n_frames) by 4 times

Closes https://github.com/kxproject/kx-audio-driver/issues/5.

It seems that on Mavericks, something (the Timer Coalescing thing?) can cause a buffer over/underrun, resulting in (wild guess!) CAEngine and the hardware getting out of sync which will make playback crackle heavily. Increasing the Engine buffer prevents this from happening.

(NB: Someone with better knowledge of OSX Kernel/Driver/CoreAudio and the E-mu hardware should probably fix this properly, but I have a feeling the former as well as (and especially) the latter are getting really rare nowadays due to people rather sticking to motherboard audio et al...)