Many vector layers flash brighter when transitioning levels during zoom

[rsargent]

Parent and child tiles are sometimes drawn at the same time during zoom, when a parent is loaded and fewer than all 4 children are loaded. The parent and partial children are all drawn. For raster without transparency, the child covers the parent, for a good result. But with vectors or alpha, sometimes lines don't line up (e.g. WDPA) or things become brighter (e.g. dotmaps). Ideally we should either not draw a partial set of children when the parent is present, or draw partial parent where children don't overlap . The latter might be unworkable.