Sample rate conversion doesn't perform well

[thijzert]

When running speeldoos on a Raspberry Pi 1, having a mix of 48kHz and 44.1kHz audio in your library isn't the greatest experience. When it tries to play something it has to convert from one to the other, you get a small chunk of audio every 10 seconds or so. Playing a 20-minute file takes over 4 hours.

The hardware in the Pi 1 is rather constrained: it's a 32-bit single-core board. Having it do 672,000 sine calculations per second probably isn't its idea of "fun".

There's bound to be some clever way of doing all the floating-point operations ahead of time, and reducing that to a multiplication table on int64's, using fixed-point math. On a Pi 1 it's still not going to be quick, but if this makes it faster than realtime it'll make all the difference.

[thijzert]

I added a benchmark to measure just how bad it is: converting 100 seconds of audio takes 28 minutes, so we're reading audio at 0.06× speed.