Fix various GCC warnings

- fix overload hiding
- handle missing enumeration values in switch
- remove extraenous semicolons
- always have defaults in color converter switch
- fix root cause of stringop warning

source/Irrlicht/COpenGLDriver.h

@@ -148,6 +148,9 @@ namespace video
 			const core::rect<s32>& sourceRect, const core::rect<s32>* clipRect = 0,
 			const video::SColor* const colors = 0, bool useAlphaChannelOfTexture = false) _IRR_OVERRIDE_;
 
+		// Explicitly bring in base class methods, otherwise
+		// this overload would hide them.
+		using CNullDriver::draw2DImage;
 		virtual void draw2DImage(const video::ITexture* texture, u32 layer, bool flip);
 
 		//! draws a set of 2d images, using a color and the alpha channel of the