This replaces *_NewMap with *_NewScene and adds SCR_NewScene to handle
loading a new map (for quake) in the renderer, and will eventually be
how any new scene is loaded.
r_screen isn't really the right place, but it gets the scene rendering
out of the low-level renderers and will make it easier to sort out
later, and hopefully easier to figure out a good design for vulkan.
Multiple render passes are needed for supporting shadow mapping, and
this is a huge step towards breaking the Vulkan render free of Quake,
and hopefully will lead the way for breaking the GL renderers free as
well.
