This commit is intended to just experiment with the idea and only
supports f32 -> s16 conversions.

The rationale behind this commit is to ensure the size of each data
structure remains consistent regardless of which backends are enabled
and disabled. This is important for mini_al because it publicly exposes
every struct.

The rationale for this is that it's to unreliable and inconsistent
across each backend and not worth the added maintenance cost.

These changes add a mal_context parameter and moves the output device
to the last parameter for consistency with mal_context_init().