Fixed pop-up menu and commander UI cursor centering

[RGreenlees]

The pop-up menu and commander UI now centre the mouse cursor when opening, making it less confusing to use.

[pierow]

If we do this it'll fix a bug with showcursor decrementing outside of a useful range when m_rawinput == 0

                    if (this->mFramesSinceEnteredTopdownMode > 2)
                    {
                        // Workaround for not being able to center mouse with raw input enabled.
                        if (CVAR_GET_FLOAT("m_rawinput") != 0)
                        {
                            if (SDL_GetRelativeMouseMode() != SDL_TRUE)
                            {
                                SDL_SetRelativeMouseMode(SDL_TRUE);

                            }
                        }
#ifdef WIN32
                        ShowCursor(TRUE);
#endif
                    }
                    else
                    {
                        this->mFramesSinceEnteredTopdownMode++;
                    }

[RGreenlees]

Thanks Pierow, I've fixed that with a second commit on this PR