RFR: 8260528: Clean glass-gtk sizing and positioning code

[Pankaj Bansal]

On Sun, 21 Mar 2021 00:31:35 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> Looks good now.
>
> Testing:
> 
> **master**
> ![master](https://user-images.githubusercontent.com/30704286/111889893-6e793f80-89c3-11eb-8f38-640e8123794c.png)
> 
> **tsayao:glass_gtk_new_position_and_size**
> ![branch](https://user-images.githubusercontent.com/30704286/111889897-733df380-89c3-11eb-9929-2ed55471638e.png)
> 
> `--tests test.robot.javafx.scene.RobotTest` causes some intermittent error but eventually all passes.

I ran the full test on Ubuntu 20.10 and the results look fine. I do not see any new failure.

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PR: https://git.openjdk.java.net/jfx/pull/367