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/cocoa/cocoa_v.h

@@ -76,6 +76,7 @@ protected:
 	float GetDPIScale() override;
 	void InputLoop() override;
 	void Paint() override;
+	void CheckPaletteAnim() override;
 
 private:
 	bool PollEvent();
@@ -88,7 +89,6 @@ private:
 	bool MakeWindow(int width, int height);
 
 	void UpdatePalette(uint first_color, uint num_colors);
-	void CheckPaletteAnim();
 
 	void BlitIndexedToView32(int left, int top, int right, int bottom);
 };