automatically adjust resolution for overview for hi-res displays

The overview window becomes less useful when working with large alignments on on hi-res displays because it renders the alignment according to the screen's physical resolution. That means that each cell in the overview is too small to perceive by eye, so detail is effectively lost - particularly for large alignments.

The simplest fix is to make sure the overview uses the OS's screen-resolution scaling factor when computing the pixel size for the overview.

[ A future fix would be to use a window based subsampling algorithm to ensure key patterns of shading are rendered in the overview, and going further.. implement semantic zooming for the alignment overview! ]