Finish up ATI_separate_stencil

Add entrypoints to glapi XML file and regenerate files.
Implement glStencilOpSeparateATI().
Consolidate some code in stencil.c

src/mesa/main/state.c

@@ -817,6 +817,10 @@ _mesa_init_exec_table(struct _glapi_table *exec)
 #if FEATURE_EXT_framebuffer_object
    SET_FramebufferTextureLayerEXT(exec, _mesa_FramebufferTextureLayerEXT);
 #endif
+
+   /* GL_ATI_separate_stencil */
+   SET_StencilFuncSeparateATI(exec, _mesa_StencilFuncSeparateATI);
+   SET_StencilOpSeparateATI(exec, _mesa_StencilOpSeparateATI);
 }