gallium/util: add shader for stencil-blits

gallium/util: add shader for stencil-blits

This shader is useful to replicate single bit from a stencil buffer even
when there's no support for PIPE_CAP_SHADER_STENCIL_EXPORT.

This is useful for the D3D12 driver, where the graphics pipeline is the
only way of writing to MSAA stencil-buffers, and not all drivers support
exporting the stencil-value from the shader.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6681>