provide commonly used vars to function-based style properties by placing them in scope
this is done by wrapping the function in another function that establishes the scope
need to fully re-parse styles in worker before rebuilding
add support for custom properties per style-rule, that can be exposed to function-based filters, colors, etc.
rename Scene.rebuildTiles() to just Scene.rebuild()
rename Scene.reloadScene() to Scene.reload()
change default workers to 2
prevent race conditions when render modes are re-compiled multiple times
Merge branch 'master' into stylefuncs
avoid initial failure to initialize data source in web worker
Merge pull request #47 from tangrams/stylefuncs

Add custom properties to function-based styles, and simplify method signatures
Merge branch 'master' into sprites