st/mesa: enable stencil shader export extension if supported

src/mesa/state_tracker/st_extensions.c

@@ -435,4 +435,8 @@ void st_init_extensions(struct st_context *st)
    if (screen->get_param(screen, PIPE_CAP_DEPTH_CLAMP)) {
       ctx->Extensions.ARB_depth_clamp = GL_TRUE;
    }
+
+   if (screen->get_param(screen, PIPE_CAP_SHADER_STENCIL_EXPORT)) {
+      ctx->Extensions.ARB_shader_stencil_export = GL_TRUE;
+   }
 }