Place GraphicsResources functions in header file

These are functions used in other files that are not on the
GraphicsResources class but are implemented inside the
GraphicsResources.cpp file. For some reason they were never put in the
GraphicsResources.h file until now, even though it's a perfectly good
header file to put them in.

desktop_version/src/GraphicsResources.h

@@ -50,4 +50,10 @@ public:
     SDL_Texture* im_flipsprites_translated;
 };
 
+SDL_Surface* LoadImageSurface(const char* filename);
+SDL_Texture* LoadImage(const char *filename, TextureLoadType loadtype);
+
+bool SaveImage(const SDL_Surface* surface, const char* filename);
+bool SaveScreenshot(void);
+
 #endif /* GRAPHICSRESOURCES_H */