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

[Thiago Milczarek Sayao]

On Sat, 6 Mar 2021 23:53:06 GMT, Thiago Milczarek Sayao <tsayao at openjdk.org> wrote:

>> Changed to WIP while we run some more tests to make sure latest changes are ok.
>> I'm fixing some residual "extra resize" issues (these issues exists on the current versions, so this makes it better, hopefully).
>
> 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.

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