Allow the user to set the background clear color during emulation

The background color can be seen at the sides of the bottom screen or when the window is wider than normal.

src/core/settings.h

@@ -44,6 +44,11 @@ struct Values {
     // System Region
     int region_value;
 
+    // Renderer
+    float bg_red;
+    float bg_green;
+    float bg_blue;
+
     std::string log_filter;
 } extern values;