Dynamically load libpulse.so.0 and libasound.so.1 on Linux

By generating stubs using https://github.com/hpvb/dynload-wrapper we
can dynamically load libpulse and libasound on systems where it is available.
Both are still a build-time requirement but no longer a run-time dependency.

For maintenance purposes the wrappers should not need to be re-generated
unless we want to bump pulse or asound to an incompatible version. It is
unlikely we will want to do this any time soon.

This closes #20978

drivers/pulseaudio/SCsub

@@ -2,4 +2,7 @@
 
 Import("env")
 
+if "pulseaudio" in env and env["pulseaudio"]:
+    env.add_source_files(env.drivers_sources, "pulse-so_wrap.c")
+
 env.add_source_files(env.drivers_sources, "*.cpp")

drivers/pulseaudio/audio_driver_pulseaudio.cpp

@@ -233,6 +233,10 @@ Error AudioDriverPulseAudio::init_device() {
 }
 
 Error AudioDriverPulseAudio::init() {
+	if (initialize_pulse()) {
+		return ERR_CANT_OPEN;
+	}
+
 	active = false;
 	thread_exited = false;
 	exit_thread = false;

drivers/pulseaudio/audio_driver_pulseaudio.h

@@ -37,7 +37,7 @@
 #include "core/os/thread.h"
 #include "servers/audio_server.h"
 
-#include <pulse/pulseaudio.h>
+#include "pulse-so_wrap.h"
 
 class AudioDriverPulseAudio : public AudioDriver {
 	Thread thread;