Port of https://github.com/mapbox/mapbox-gl-js/pull/5101: adds a new render pass `Pass3D` before any other rendering wherein we render layers with 3D passes (fill-extrusion layers) to offscreen framebuffers, sharing a depth renderbuffer between those layers in order to render 3D space correctly. Those framebuffers are saved on the RenderLayers and copied back to the map during the translucent pass. Rendering to offscreen framebuffers before we do any clear + draw means we can avoid expensive framebuffer restores.
- AnnotationData.updateData() was not completely thread safe. Calling it from the renderer thread instead of the main thread fixes this
- Since we moved the coalescing of updates out of the map and into the renderer frontend implementations, the updateData was called too much, resulting in degraded performance. Moving it into the renderer makes sure it gets coalesced again.
- Make renderer frontend responsible for creating a BackendScope on render
- encapsulate backend scope handling on cleanup and low memory in renderer impl
