[FIX] compiler: do not look up ComponentNode in the context

With the introduction of t-call-context, there is now no guarantee that
the ComponentNode can be found in the rendering context, any attempt to
do so can crash when combined with t-call-context. This commit fixes
that by using `this` instead, which in compiled templates refers to the
component that is being rendered.

tests/compiler/t_ref.test.ts

@@ -114,7 +114,8 @@ describe("t-ref", () => {
     app.addTemplate("main", main);
     app.addTemplate("sub", sub);
 
-    const bdom = app.getTemplate("main")({ __owl__: { refs } }, {});
+    const comp = { __owl__: { refs } };
+    const bdom = app.getTemplate("main").call(comp, comp, {});
     mount(bdom, document.createElement("div"));
 
     expect(refs.name.tagName).toBe("SPAN");