cogl/pipeline/glsl: Distinguish between pipeline state types

Both the fragend and vertend shader state was called
"CoglPipelineShaderState", which was rather annoying, especially when
the type needs to be exposed outside of the .c file as part of moving
out unit tests. Make the types unique. This also avoids confusing what
type one is looking at.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2555>