add semi-portable looping using fsnotify

generic defaultLoopRender for use in Backends
Relax process bounds for older platforms.
Add debugging output
shorter sleep delay on MacOS

to work around https://ghc.haskell.org/trac/ghc/ticket/7325

fix lower bound on fsnotify—required for polling interval
Merge pull request #213 from diagrams/fsnotify

add semi-portable looping using fsnotify