This is part of some future work to decouple `ma_context` and
`ma_device` from backends. If you need access to the context for some
reason, you can pass it in via the user data.
With this commit, custom backends can now be implemented as
self-contained modules that can easily be plugged in and mixed and
matched depending on a programs requirements. The order in which
custom backends are specified in the context config determine their
priority.
This commit updates the custom_backend example by moving the SDL
code out into its own file in "extras/backends/sdl". The example will
now just include the SDL code files like normal. This represents a more
realistic scenario.
David Reid