RFR: 8360940: Layout stops updating when using (public api) Parent#setNeedsLayout(true) due to incorrect state management

[John Hendrikx]

Ensures proper propagation of layout flags when using `forceParentLayout = true`.

This was the root cause of issue #1874

### Note

Apparently it is still quite easy to mess up the layout flags. Basically, the layout flag tracked by Parent should always either be `CLEAN` for any scene graph branch, or `!CLEAN` + a layout pulse is scheduled on the corresponding `Scene`.

However, with careful use of the public API `requestLayout` one can get these flags in a bad state still:

Let's say I have a branch `A (root node under Scene) -> B -> C`, **and** a layout is in progress, **and** we're currently in the `layoutChildren` method of `C`. The flag `performingLayout` will be `true` for all nodes in the branch `A` ->  `B` -> `C`.  The `layout` method will set the layout flag to `CLEAN` as its first action, so when we're at `C::layoutChildren`, all flags have been reset to `CLEAN` already.  See the `Parent::layout` method for how all this works.

Now, to mess up the flags, all you need to do is call `requestLayout` on `B` or `C` from the `layoutChildren` of `C` (or indirectly by changing something and something is listening to this and schedules a layout on something somewhere in this branch); note that `requestLayout` is not documented to be illegal to call during layout, and some classes in FX will do so (ScrollPaneSkin, NumberAxis, etc..) risking the flags getting in a bad state... -- usually you get away with this, as there are many ways that layout is triggered, and eventually the flags will get overwritten and reset to a consistent state.

The bad state occurs because this code path is followed (all code from `Parent`):

    public void requestLayout() {
        clearSizeCache();
        markDirtyLayout(false, forceParentLayout);
    }
    
Calls to `markDirtyLayout(false, false)`:

    private void markDirtyLayout(boolean local, boolean forceParentLayout) {
        setLayoutFlag(LayoutFlags.NEEDS_LAYOUT);
        if (local || layoutRoot) {
            if (sceneRoot) {
                Toolkit.getToolkit().requestNextPulse();
                if (getSubScene() != null) {
                    getSubScene().setDirtyLayout(this);
                }
            } else {
                markDirtyLayoutBranch();
            }
        } else {
            requestParentLayout(forceParentLayout);
        }
    }
    
Before going into the `else` (as none of the nodes is a layout root, and `local` was set to `false`) it will do **setLayoutFlag(LayoutFlags.NEEDS_LAYOUT)** -- this will set a flag on some node; to eventually end up in a consistent state, it must mark all ancestors as well and schedule a pulse (with `Toolkit.getToolkit().requestNextPulse()`)... but:

    void requestParentLayout(boolean forceParentLayout) {
        if (!layoutRoot) {
            final Parent p = getParent();
            if (p != null && (!p.performingLayout || forceParentLayout)) {

                /*
                 * The forceParentLayout flag must be propagated to mark all ancestors
                 * as needing layout. Failure to do so while performingLayout is true 
                 * would stop the propagation mid-tree. This leaves some nodes as needing 
                 * layout, while its ancestors are clean, which is an inconsistent state.
                 */

                p.requestLayout(forceParentLayout);
            }
        }
    }
    
Here there is a guard `!p.isPerformingLayout`, blocking propagation up the tree.  As said, this flag is `true` for all nodes during a layout of the same branch.  The end result thus is that some nodes have their layout flag changed to `NEEDS_LAYOUT`, but it was not propagated, nor was a pulse scheduled...

-------------

Commit messages:
 - Remove trailing whitespace
 - Propagate forceParentLayout flag to prevent inconsistent layout state

Changes: https://git.openjdk.org/jfx/pull/1879/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1879&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360940
  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1879.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1879/head:pull/1879

PR: https://git.openjdk.org/jfx/pull/1879