cube: Support runtime selection of WSI platform

[charles-lunarg]

By changing the selection of a WSI platform from a build time choice to a runtime choice, this allows the removal of vkcube-wayland as a separate binary.

Use --wsi <xlib|xcb|wayland|display|directfb> to choose the WSI platform that vkcube will render with. The chosen platform must be one that vkcube was compiled with support for, so directfb will be unavailable without changing the build parameters to enable it.

These changes have been made to both vkcube and vkcubepp.

Platforms which do not have multiple WSI platforms, such as windows, android, and apple (metal is the only non-deprecated platform) do not have this option as there aren't any platforms to choose from.