Factorize software renderer backend switching warning to be fetched from a new GetWarningMessage in video backend - will be needed for DX11.1 feature set warnings

Source/Core/VideoCommon/VideoBackendBase.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -42,6 +43,7 @@ public:
   virtual std::string GetName() const = 0;
   virtual std::string GetDisplayName() const { return GetName(); }
   virtual void InitBackendInfo() = 0;
+  virtual std::optional<std::string> GetWarningMessage() const { return {}; }
 
   // Prepares a native window for rendering. This is called on the main thread, or the
   // thread which owns the window.