This commit also fixes some bugs in the WASAPI backend.

This includes changes to callbacks used by custom backends. It adds a
`pConfig` parameter to both onContextInit and onDeviceInit. This allows
custom backends to specify custom config properties.

With the introduction of custom backends, it became necessary to
simplify the backend callbacks. For backwards compatibility, all
built-in backends are still using the old callbacks. Future commits are
going to be transitioning built-in backends to the new system. This
commit is designed to help with this migration so that we can migrate
backends one at a time while still keeping compatibility with this old
system, thereby allowing backends that haven't yet transitioned to keep
working.

This is adding too much complexity, is less efficient than doing it
manually, and is not compatible with some future changes for supporting
effects that require multiple input streams.

This is in preparation for some future work.

This has been replaced with a simpler fading system.

Public issue https://github.com/mackron/miniaudio/issues/221

Fix intermediaryBufferView losing reference to wasm memory after heap resize

Fix compiler warnings with GCC 4.7

Fix C++ compilation errors with the PulseAudio backend.

C++ does not allow implicit casts from void*, so add explict casts.

Issue: https://github.com/mackron/miniaudio/issues/215