Ensure _localMatMdf is reset to false

With efeb109, support for transform effects has been added, which
required introducing another matrix on `TransformElement` to represent
the final transform matrix including the effects. This matrix is denoted
as `localMat`

If there are no transform effects, `localMat` is the same
(referentially) as the existing transform matrix (`mat`). Otherwise,
it's calculated by composing the transform effects and then composing
that with the "old" transform matrix.

Alongside `localMat`, a `_localMatMdf` member is introduced, which
tracks whether the local matrix was modified since its last commit to
the DOM, analogously to `_matMdf`.

Unfortunately, a logic bug introduced by efeb109 meant that, in the
absence of transform effects, `_localMatMdf` was latched to `true`,
i.e. no code path would ever be able to reset it to `false` after it was
once set to `true` (e.g. as a result of the matrix being animated).

This causes a serious performance regression, since all matrices that
once had `_matMdf` set to `true` will keep being updated in the DOM
every frame, causing costly full style recalculations.

Fix by forcing `_matMdf` and `_localMatMdf` to be equivalent in the
case of no transform effects, just as `mat` and `localMat` are
referentially equal. In the case of >0 transform effects, the custom
logic in `renderLocalTransform()` will take over and compute
`_localMatMdf` separately from and based on `_matMdf`.

player/js/elements/helpers/TransformElement.js

@@ -53,7 +53,7 @@ TransformElement.prototype = {
         }
       }
     }
-    if (this.finalTransform._matMdf) {
+    if (!this.localTransforms || this.finalTransform._matMdf) {
       this.finalTransform._localMatMdf = this.finalTransform._matMdf;
     }
     if (this.finalTransform._opMdf) {