fix warnings (#648)

* fix unknown command line argument

* fix warnings settings for gcc

* fix -Wzero-as-null-pointer-constant

* revert CMakeLists.txt

src/graphics/core/Cubemap.cpp

@@ -25,7 +25,7 @@ Cubemap::Cubemap(uint width, uint height, ImageFormat imageFormat)
             0, 
             format, 
             GL_UNSIGNED_BYTE, 
-            NULL
+            nullptr
         );
     }
 }

src/graphics/core/Shadows.cpp

@@ -23,7 +23,7 @@ public:
         glGenTextures(1, &depthMap);
         glBindTexture(GL_TEXTURE_2D, depthMap);
         glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT,
-            resolution, resolution, 0, GL_DEPTH_COMPONENT, GL_FLOAT, NULL);
+            resolution, resolution, 0, GL_DEPTH_COMPONENT, GL_FLOAT, nullptr);
         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);

src/graphics/ui/gui_util.hpp

@@ -17,7 +17,7 @@ namespace guiutil {
     /// @brief Create element from XML
     /// @param source XML
     std::shared_ptr<gui::UINode> create(
-        gui::GUI& gui, const std::string& source, scriptenv env = 0
+        gui::GUI& gui, const std::string& source, scriptenv env = nullptr
     );
 
     void alert(