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/allegro_v.h

@@ -36,6 +36,7 @@ public:
 protected:
 	void InputLoop() override;
 	void Paint() override;
+	void CheckPaletteAnim() override;
 };
 
 /** Factory for the allegro video driver. */