Follow cl-slug 8579836: the pixel shader is cl-slug/clsl, in clsl
The shader pages, the systems, the example that draws a ring on the GPU
and the requirements name clsl and cl-slug/clsl in place of cl-gsl and
cl-slug/gsl; the examples are in clsl's language, the ring's run as
shown. Vertex shaders take Vulkan's clip space, y down, which cl-gfx
and clsl use. A program that calls SLUG-COVERAGE needs the device
feature that keeps NaNs, as PROGRAM-FEATURES reports.
