* Add missing stdlib headers
* Replace msvc-specific std::ios constant with the standard's equivalent

Automatic conversion from float to some other type isn't always desired.

This allows new audio blocks to be prepared while the old ones are
processed by the audio device.

This cache is unlikely to be used in practice anyway.

This also prevents valgrind from whining about possibly lost data. It may
still emit messages about "still reachable" data if the PulseAudio plugin
is used for playback, but these are a lesser concern.

ALSA backend

SDL2_Mixer backend implementation