Streamlined Overview Creation

Xash3D creates overview files at the push of a button. With dev_overviev set to 1, configure the display options as necessary, and press the screenshot button. Xash3D will automatically create a screenshot, compress the input colorspace to 8 bits and write out the overview script. Of course, the legacy option to create overviews as in the original Half-Life has been preserved—for that, use "dev_overview" "2".