Fix skybox Cubemaps

Previously we had skyboxes and cubemaps loading in, but I never got around to adding them in a proper Subpass now that we have the rendering refactor completed.

See `Cubemap.h` and `Cubemap.cpp` for examples on how we loaded them in before.

I want to change this so that you don't have to pass in so many arguments to the Cubemap class, but need to do some more reading on the best ways to implement this in Vulkan specifically.