Now that ref/openal ships a current alext.h, include it and use the real
LPALC*SOFT / ALCEVENTPROCTYPESOFT typedefs instead of hand-rolled function
pointer types and #define'd ALC_* tokens. The event callback is marked
ALC_APIENTRY + noexcept to match ALCEVENTPROCTYPESOFT. No behaviour change.
Replace the ~1 Hz poll (which relied on alcGetString(DEFAULT_ALL_DEVICES),
whose value OpenAL caches and does not refresh on re-plug) with an
alcEventCallbackSOFT handler for DEFAULT_DEVICE_CHANGED / DEVICE_REMOVED. The
callback (possibly off-thread) just flags the change; update() reopens playback
on the new default on the main thread. Now both unplugging and re-plugging the
output are followed. Falls back to the ALC_CONNECTED poll when system_events is
unavailable, and only chases the default when no device is pinned.
OpenAL does not re-route on its own, so unplugging the active output device
(e.g. headphones) mid-game left the sim silent until restart. Poll ALC_CONNECTED
(ALC_EXT_disconnect) at ~1 Hz and, when the device is lost, reopen playback on
the current default output via alcReopenDeviceSOFT, preserving the context and
all sources. Tokens are declared locally since the bundled AL headers ship no
alext.h; the entry point is resolved at runtime, so this is a no-op (with a log
warning) on OpenAL Soft builds too old to provide ALC_SOFT_reopen_device.
