Make viewport safe for the Steam Deck (and 720p displays)

Addresses issue as described here: https://www.loverslab.com/topic/231570-libidinous-loader-rjw-mod-loader-and-providers-list-100-mods-supported/page/5/#findComment-6923286

Changes the VIEWPORT_HEIGHT to 640 from 800, which would overflow on displays utilizing 1280x720 and 1280x800 pixel resolutions. 640 was chosen to account for the roughly 80 pixels used by Plasma for its default panel, as well as the window decorations applied to the loader such as the title bar.