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/win32_v.h

@@ -49,12 +49,11 @@ protected:
 	void UnlockVideoBuffer() override;
 	void Paint() override;
 	void PaintThread() override;
+	void CheckPaletteAnim() override;
 
 private:
 	std::unique_lock<std::recursive_mutex> draw_lock;
 
-	void CheckPaletteAnim();
-
 	static void PaintThreadThunk(VideoDriver_Win32 *drv);
 };