llvmpipe: add reference counting to fragment shaders.

llvmpipe: add reference counting to fragment shaders.

Currently llvmpipe calls finish on the context when a shader
variant has to be destroyed just in case the variant is currently
in use by the setup engine.

Fix this by reference counting the shaders, and reference counting
the shader variants.

Whenever a shader is used in the rasteriser backend, it is added
to a reference list and removed when the rasterizer is finished with it.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6341>