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
         );
     }
 }