RFR: 8264591: HBox/VBox child widths pixel-snap to wrong value

[mstr2]

The children of HBox/VBox don't always pixel-snap to the same value as the container itself when a render scale other than 1 is used. This can lead to a visual glitch where the content bounds don't line up with the container bounds. In this case, the container will extend beyond its content, or vice versa.

The following program shows the problem for HBox:
Region r1 = new Region();
Region r2 = new Region();
Region r3 = new Region();
Region r4 = new Region();
Region r5 = new Region();
Region r6 = new Region();
r1.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
r2.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
r3.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
r4.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
r5.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
r6.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
r1.setPrefWidth(25.3);
r2.setPrefWidth(25.3);
r3.setPrefWidth(25.4);
r4.setPrefWidth(25.3);
r5.setPrefWidth(25.3);
r6.setPrefWidth(25.4);
r1.setMaxHeight(30);
r2.setMaxHeight(30);
r3.setMaxHeight(30);
r4.setMaxHeight(30);
r5.setMaxHeight(30);
r6.setMaxHeight(30);
HBox hbox1 = new HBox(r1, r2, r3, r4, r5, r6);
hbox1.setBackground(new Background(new BackgroundFill(Color.RED, null, null)));
hbox1.setPrefHeight(40);

r1 = new Region();
r2 = new Region();
r3 = new Region();
r4 = new Region();
r5 = new Region();
r6 = new Region();
r1.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
r2.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
r3.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
r4.setBackground(new Background(new BackgroundFill(Color.GREY, null, null)));
r5.setBackground(new Background(new BackgroundFill(Color.DARKGRAY, null, null)));
r6.setBackground(new Background(new BackgroundFill(Color.BLACK, null, null)));
r1.setPrefWidth(25.3);
r2.setPrefWidth(25.3);
r3.setPrefWidth(25.4);
r4.setPrefWidth(25.3);
r5.setPrefWidth(25.3);
r6.setPrefWidth(25.4);
r1.setMaxHeight(30);
r2.setMaxHeight(30);
r3.setMaxHeight(30);
r4.setMaxHeight(30);
r5.setMaxHeight(30);
r6.setMaxHeight(30);
HBox hbox2 = new HBox(r1, r2, r3, r4, r5, r6);
hbox2.setBackground(new Background(new BackgroundFill(Color.RED, null, null)));
hbox2.setPrefHeight(40);
hbox2.setPrefWidth(152);

VBox root = new VBox(new HBox(hbox1), new HBox(hbox2));
root.setSpacing(20);
Scene scene = new Scene(root, 500, 250);

primaryStage.setScene(scene);
primaryStage.show();

Here's a before-and-after comparison of the program above after applying the fix. Note that 'before', the backgrounds of the container (red) and its content (black) don't align perfectly. The render scale is 175% in this example.
![pixel-glitch](https://user-images.githubusercontent.com/43553916/112891996-146e2d80-90d9-11eb-9d83-97754ae38dc1.png)

I've filed a bug report and will change the title of the GitHub issue as soon as it's posted.

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

Commit messages:
 - Fix pixel-snapping glitches in VBox/HBox
 - Failing test

Changes: https://git.openjdk.java.net/jfx/pull/445/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=445&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264591
  Stats: 306 lines in 5 files changed: 215 ins; 19 del; 72 mod
  Patch: https://git.openjdk.java.net/jfx/pull/445.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/445/head:pull/445

PR: https://git.openjdk.java.net/jfx/pull/445