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.
- Make renderer frontend responsible for creating a BackendScope on render
- encapsulate backend scope handling on cleanup and low memory in renderer impl
- Guards against duplicate activations by checking wether the backend of the prior scope is the same as the current ones
- Makes sure that only the most outer backend scope deactivates by tracking activation state
