Add snapshot() to write the hosted window as a PNG

[yannickpulver]

Summary

• Adds snapshot("name"), which writes the hosted window as a PNG. Useful for looking at a view in a PR review.
• Includes menu and popover windows. Sheets show up with their dimmed backdrop.
• Writes to SNAPSHOT_DIR (or a temp folder). Compares nothing and leaves out the system status bar.
• Falls back to layer rendering without a host app. There drawHierarchy draws an empty image.
• Documents in the README that the host app needs a launch screen. Without one iOS runs it at 320×480 points.