[openal] Convolution reverb

[Chris Robinson]

On Wed, 26 Aug 2020 22:48:18 +0300
Sean <s.tolstoyevski at gmail.com> wrote:

> It's a great feature.
> 
> But the example is too big. It will be very complex to understand if
> we are not an experienced C++ coder.
> 
> Would you accept PR if I translated it to C?

I redid the example based on the alstream.c example. See here for the
updated version:

https://github.com/kcat/openal-soft/blob/97ecf581/examples/alconvolve.c