Codechange: be consistent in what CheckPaletteAnim() does and when it is called

Additionally, make sure this is a class method. Later commits
will make use of this.

src/video/sdl_v.h

@@ -43,6 +43,7 @@ protected:
 	void UnlockVideoBuffer() override;
 	void Paint() override;
 	void PaintThread() override;
+	void CheckPaletteAnim();
 
 private:
 	std::unique_lock<std::recursive_mutex> draw_lock;