Sticky pinning now works on retina and high-resolution screens

Sticky headers, nav bars and table headers now pin correctly on the displays where the previous release could not yet show them: retina Macs and high-resolution Windows screens. A pinned element is drawn as its own layer that the display keeps fixed to the top of the window as you scroll, so it holds its place at full smoothness without the page being redrawn underneath it. On retina Macs, where Floati draws the page straight on the graphics card, the pinned element is now a real composited layer that moves with no extra drawing per scroll. High-resolution Windows screens, which show the page as image tiles, now get the pinned element as a separate overlay the window keeps in place. With this, sticky page furniture behaves the same on every platform and every display, matching the reference browser, and scrolling stays smooth on long pages.