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.
Finally. I never liked it (felt bad adding it in the first place), and
it has caused confusion with function and global variable names, but it
did let me get the render plugins working.
