More work on custom device backends.

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.

extras/backends/sdl/backend_sdl.h

@@ -0,0 +1,34 @@
+/*
+The SDL backend does not require any user data, nor configs. Configs are provided here in case
+they are needed in the future, however you can safely pass in NULL when setting up your context
+and device configs.
+*/
+#ifndef miniaudio_backend_sdl_h
+#define miniaudio_backend_sdl_h
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+MA_API const ma_device_backend_vtable* MA_DEVICE_BACKEND_VTABLE_SDL;
+
+
+typedef struct
+{
+    int _unused;
+} ma_context_config_sdl;
+
+MA_API ma_context_config_sdl ma_context_config_sdl_init(void);
+
+
+typedef struct
+{
+    int _unused;
+} ma_device_config_sdl;
+
+MA_API ma_device_config_sdl ma_device_config_sdl_init(void);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* miniaudio_backend_sdl_h */