Viewport shenanigans

Not a lot of substance here, but play with the viewport/Xshm shim so the desktop demos (Weston/sway) do the right thing on master, and also to demonstrate that we can render fullscreen to a 2400x1600 framebuffer, blowing the previous cap (2048x1280) out of the water and letting us do the HiDPI needed for actual desktops on Kevin.