Abort layer drawing early

[kkaefer]

Now that we're drawing layer by layer (#122), we need a way to abort early when the current layer is not being drawn, e.g. because the stylesheet doesn't show it, or because none of the tiles on the layer has any data. This allows us to not update the clip id for every tile and iterating through all tiles in general.

• abort if a tile has no data
• abort if the style specifies "disabled"
• abort if the current render pass wouldn't render anything for a bucket type