This paves the way for clean initialization of the Vulkan renderer, and
very much cleans up the older renderer initialization code as gl and sw
are no longer intertwined.
A single graphics-capable queue should be enough for now. However, I'm
not sure I'm happy with a lot of the code: it's a bit difficult to write
flexibly configured code for Vulkan (or seems to be at this stage),
especially in C.
Bill Currie