Polyscope

Adds high-performance mode for big point clouds, volume mesh inspection with slice planes, more slice plane config options, better control of structure transformations and scene extents, orthographic view, back face coloring options, advanced options for customizing ImGui, support for shared linking, and bug fixes.

Minor bugfixes to view management, up directions, and handling of show()

Select backend at runtime, improve backend support in build system, keep window invisible until first show, bugfixes, add Windows builds to CI.

Overhaul render backend, extended testing, and more.