Fix for incorrectly using clamped resolution when looking for framerate match

The resolution structure contains iWidth/iHeight which is the size of the (possibly clamped) GUI overlay, and iScreenWidth/iScreenHeight which is the size of the physical display (and the destination size videos will be scaled to).

The framerate matching code is incorrectly using the GUI overlay resolution size to find the best matching HDMI mode.

On the Pi all modes bigger than 1280x720 have iWidth=1280 and iHeight=720, so you can end up choosing a 720p display mode when playing a 1080p video with a 1080p display mode selected.