Eliminate duplicate style validation during style loading

[jfirebaugh]

In looking into #2991, I noticed duplicate work being put into style validation on the main thread during startup. First, the entire style is validated via validateStyle(stylesheet). Then, each individual source, layer, and property is re-validated, via addSource and StyleLayer.create. The revalidation is unnecessary duplicate work. After validating the complete style up front, style loading should use code pathways for initializing sources and layers that don't perform the validation that are required by the public runtime styling APIs.