[FIX] qweb: protect scope in t-call writing recursively

Have something like
```xml
 <div t-name="Parent">
   <t t-call="nodeTemplate">
     <t t-set="recursive_idx" t-value="1"/>
     <t t-set="node" t-value="root"/>
   </t>
 </div>

 <div t-name="nodeTemplate">
   <t t-set="recursive_idx" t-value="recursive_idx + 1"/>
   <p><t t-esc="node.val"/> <t t-esc="recursive_idx"/></p>
   <t t-foreach="node.children or []" t-as="subtree">
     <t t-call="nodeTemplate">
       <t t-set="node" t-value="subtree"/>
     </t>
   </t>
 </div>
```

Where we want to propagate a recursion index through recursive t-calls

Before this commit, it did not work as we protected the scope in order
to not leak, in the wronf manner. Namely the protected scope only took
firt level prototype properties of the original scope.

After this commit, this case works as we mark the scope as read only

solves #672

src/qweb/compilation_context.ts

@@ -196,11 +196,12 @@ export class CompilationContext {
     const protectID = this.generateID();
     this.rootContext.protectedScopeNumber++;
     this.rootContext.shouldDefineScope = true;
-    const scopeExpr = codeBlock
-      ? `Object.create(scope);`
-      : `Object.assign(Object.create(context), scope);`;
+    const scopeExpr = `Object.create(scope);`;
     this.addLine(`let _origScope${protectID} = scope;`);
     this.addLine(`scope = ${scopeExpr}`);
+    if (!codeBlock) {
+      this.addLine(`scope.__access_mode__ = 'ro';`);
+    }
     return protectID;
   }
   stopProtectScope(protectID: number) {

src/qweb/qweb.ts

@@ -133,7 +133,7 @@ const UTILS: Utils = {
   },
   getScope(obj, property: string) {
     const obj0 = obj;
-    while (obj && !obj.hasOwnProperty(property)) {
+    while (obj && !obj.hasOwnProperty(property) && !(obj.hasOwnProperty('__access_mode__') && obj.__access_mode__ === 'ro')) {
       const newObj = obj.__proto__;
       if (!newObj || isComponent(newObj)) {
         return obj0;