Add early PipeWire backend.

This is incomplete.

extras/backends/pipewire/miniaudio_pipewire.h

@@ -0,0 +1,49 @@
+/*
+The PipeWire backend depends on the libspa development packages. On Debian-based distributions,
+this can be installed with:
+
+    sudo apt install libspa-0.2-dev
+
+If using Ubuntu, this may install it in a "spa-0.2" subfolder. In this case, you might need
+to add the following to your build command:
+
+    -I/usr/include/spa-0.2
+
+Unfortunately PipeWire has a hard dependency on the above package, and because it's made up
+entirely of non-trivial inlined code, it's not possible to avoid this dependency. It's for
+this reason the PipeWire backend cannot be included in miniaudio.h since it has a requirement
+that it does not depend on external development packages.
+*/
+#ifndef miniaudio_backend_pipewire_h
+#define miniaudio_backend_pipewire_h
+
+#include "../../../miniaudio.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern ma_device_backend_vtable* ma_device_backend_pipewire;
+
+
+typedef struct
+{
+    int _unused;
+} ma_context_config_pipewire;
+
+MA_API ma_context_config_pipewire ma_context_config_pipewire_init(void);
+
+
+typedef struct
+{
+    const char* pThreadName;    /* If NULL, defaults to "miniaudio-pipewire". */
+    const char* pStreamName;    /* If NULL, defaults to "miniaudio" */
+    const char* pMediaRole;     /* If NULL, defaults to "Game". */
+} ma_device_config_pipewire;
+
+MA_API ma_device_config_pipewire ma_device_config_pipewire_init(void);
+
+#ifdef __cplusplus
+}
+#endif
+#endif  /* miniaudio_backend_pipewire_h */