vector layer: gracefully handle missing path / polygon classes (if the server doesn't send these classes in the vector tiles, but the style wants to show them)

[adam-ce]

we removed some of the geometry on the server side to reduce the amount of data. this is now in conflict with the style, resulting in popping once the data is available on higher zoom levels. for popping free rendering, we have to adjust the style. we can do that programmatically if we know which features are available on which zoom level.-- the gpu gets the style from the scheduler. verify it can be updated at any time
-- the scheduler gets the loaded style in the constructor. currently it's then sent to the gpu via signal. instead, store the style in the scheduler, and send an all transparent style to the gpu
-- create a dictionary "feature class" -> "lowest zoom level seen".
-- once tiles come in, we look at the feature class and the dictionary, dictionary[feature class] = min(dictionary[feature class], tile zoom level). if the dictionary changed, we update the gpu styles (enable all styles in the dictionary with their min level)