st/mesa: implement depth clamp

src/mesa/state_tracker/st_atom_clip.c

@@ -56,6 +56,8 @@ static void update_clip( struct st_context *st )
       }
    }
 
+   clip.depth_clamp = st->ctx->Transform.DepthClamp != GL_FALSE;
+      
    if (memcmp(&clip, &st->state.clip, sizeof(clip)) != 0) {
       st->state.clip = clip;
       cso_set_clip(st->cso_context, &clip);

src/mesa/state_tracker/st_extensions.c

@@ -397,4 +397,8 @@ void st_init_extensions(struct st_context *st)
    if (screen->get_param(screen, PIPE_CAP_GEOMETRY_SHADER4)) {
       ctx->Extensions.ARB_geometry_shader4 = GL_TRUE;
    }
+
+   if (screen->get_param(screen, PIPE_CAP_DEPTH_CLAMP)) {
+      ctx->Extensions.ARB_depth_clamp = GL_TRUE;
+   }
 }