Check if the style is completely loaded. Note: The style specified sprite would be marked as loaded even with sprite loading error (An error will be emitted via `MapLoadingError`). Sprite loading error is not fatal and we don't want it to block the map rendering, thus the function will still return `true` if style and sources are fully loaded.
Return
`true` iff the style JSON contents, the style specified sprite and sources are all loaded, otherwise returns `false`.