Fix #7884 and #7887 by implementing the first proposition.

thanks to David for the feedback

Clone and wall is not supported yet.

bug was triggered only when using controls provided by the OS like NSWindow's fullscreen button

We only considered the audio already buffered through opensles engine,
so the calculation was incorrect.

Use samples rather than bytes for calculation, and a function rather than
hardcoding 2*2 all over the place when a size in bytes is needed.

Ubuntu bug #1038303

And simplify accordingly.

With hardware FPU, FI32 was only used within the mad decoder and
internally converted to FL32. With software FPU, FI32 did not work
properly as the S16N conversion was broken (at least since VLC1.1.0).