Pre-test opening a stream for audio backends, fall back to null if not suitable.

src/audio_core/sink/sdl2_sink.h

@@ -88,10 +88,11 @@ private:
 std::vector<std::string> ListSDLSinkDevices(bool capture);
 
 /**
- * Get the reported latency for this sink.
+ * Check if this backend is suitable for use.
+ * Checks if enabled, its latency, whether it opens successfully, etc.
  *
- * @return Minimum latency for this sink.
+ * @return True is this backend is suitable, false otherwise.
  */
-u32 GetSDLLatency();
+bool IsSDLSuitable();
 
 } // namespace AudioCore::Sink