gallium: add PIPE_CAP_SHAREABLE_SHADERS

I'll let drivers figure out how to do it.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/docs/source/screen.rst

@@ -276,6 +276,8 @@ The integer capabilities:
   GL4 hardware will likely need to emulate it with a shader variant, or by
   selecting the interpolation weights with a conditional assignment
   in the shader.
+* ``PIPE_CAP_SHAREABLE_SHADERS``: Whether shader CSOs can be used by any
+  pipe_context.