This option disables device playback and capture. This is useful if you
only need mini_al's decoding and/or decoding APIs.

When dynamically adjusting the sample rate you must now set both the
input and output sample rates at the same time. This is required for
future support for specifying a ratio instead of in/out rates.

Stop warning about uninitialised variable

For some reason, the warning only appears for me if I set MAL_NO_OPENAL
and compile with -O3.

The motivation for this change is to simplify and clean up the cross-
platform part of mini_al.