fix(Viewer): trigger resized on attach

Ensure components get notified on `canvas.resized`, whenever
the Viewer instance gets attached.

test/spec/ViewerSpec.js

@@ -962,12 +962,20 @@ describe('Viewer', function() {
         // assume
         expect(viewer._container.parentNode).not.to.exist;
 
+        /* global sinon */
+        var resizedSpy = sinon.spy();
+
+        viewer.on('canvas.resized', resizedSpy);
+
         // when
         viewer.attachTo(container);
 
         // then
         expect(viewer._container.parentNode).to.equal(container);
 
+        // should trigger resized
+        expect(resizedSpy).to.have.been.called;
+
         done(err, warnings);
       });
     });