[FIX] qweb: issue when rendering twice dynamic t-calls

The previous implementation generated an id each time the template was
rendered, instead of just once per new dynamic template found.

tests/qweb/qweb.test.ts

@@ -1111,6 +1111,9 @@ describe("t-call (template calling", () => {
     expect(renderToString(qweb, "main", { template: "foo", val: "foo" })).toBe(expected);
     const expected2 = "<div><bar>quux</bar></div>";
     expect(renderToString(qweb, "main", { template: "bar", val: "quux" })).toBe(expected2);
+    // duplicate call because there was a specific bug with some id that was
+    // incremented each rendering.
+    expect(renderToString(qweb, "main", { template: "bar", val: "quux" })).toBe(expected2);
   });
 });