Use vanilla default brightness as default for brightness setting

Source/options.cpp

@@ -672,7 +672,7 @@ GraphicsOptions::GraphicsOptions()
 #endif
               { FrameRateControl::CPUSleep, N_("Limit FPS") },
           })
-    , brightness("Brightness Correction", OptionEntryFlags::Invisible, "Brightness Correction", "Brightness correction level.", 100)
+    , brightness("Brightness Correction", OptionEntryFlags::Invisible, "Brightness Correction", "Brightness correction level.", 0)
     , zoom("Zoom", OptionEntryFlags::None, N_("Zoom"), N_("Zoom on when enabled."), false)
     , colorCycling("Color Cycling", OptionEntryFlags::None, N_("Color Cycling"), N_("Color cycling effect used for water, lava, and acid animation."), true)
     , alternateNestArt("Alternate nest art", OptionEntryFlags::OnlyHellfire | OptionEntryFlags::CantChangeInGame, N_("Alternate nest art"), N_("The game will use an alternative palette for Hellfire’s nest tileset."), false)