[FIX] store: properly render, even if shouldupdate is implemented

Before this commit, an unwanted behaviour happened when using components
with shouldUpdate implemented, and store/state.

The actual problem is the following: the sub component is using a store,
and shouldupdate.  Whenever the component is rendered, it checks if there is an incoming
rendering from the context.  If that is the case, it skips the
rendering, because we actually only want to be rendered by the store
rendering (otherwise, we may run into issue with inconsistent data (more
recent data from the context, older data in the component).

However, if shouldUpdate is implemented, then the rendering coming from
the context simply does not arrive.

To fix this, we tried to just force these renderings to go through all children,
regardless of their shouldUpdate status. This actually works, but then
shouldUpdate is ignored, which is an issue in Odoo discuss.

Then, after discussing this situation, we noticed that the context/store
system is actually unsafe: the protection given by the check mentioned
above is in fact fundamentally insufficient: there are other perfectly
valid situations where a rendering can be triggered on the component,
which will bypass the check (for example, an explicit call to
this.render() or a rendering initiated by some parent component) and
cause a crash if the component is not properly defensively written.

Therefore, it is currently mandatory for all components using
context/store to be aware of that, and to protect themselves against
such situations. So, in that regard, the check is not really a
protection, it just helps hiding an unsafe situation anyway and we
decided to remove it.

Note that this is a potentially breaking change: components using a
store and some local state will now be rendered twice in some cases...

closes #799

src/context.ts

@@ -115,16 +115,6 @@ export function useContextWithCB(ctx: Context, component: Component, method): an
     __owl__.observer = new Observer();
     __owl__.observer.notifyCB = component.render.bind(component);
   }
-  const currentCB = __owl__.observer.notifyCB;
-  __owl__.observer.notifyCB = function () {
-    if (ctx.rev > mapping[id]) {
-      // in this case, the context has been updated since we were rendering
-      // last, and we do not need to render here with the observer. A
-      // rendering is coming anyway, with the correct props.
-      return;
-    }
-    currentCB();
-  };
 
   mapping[id] = 0;
   const renderFn = __owl__.renderFn;

src/store.ts

@@ -1,5 +1,4 @@
-import { Component } from "./component/component";
-import { Env } from "./component/component";
+import { Component, Env } from "./component/component";
 import { Context, useContextWithCB } from "./context";
 import { onWillUpdateProps } from "./hooks";