Headless glass platform

[Johan Vos]

Hi,

I created a branch in the jfx-sandbox repository for experimenting with a
headless glass platform:
https://github.com/openjdk/jfx-sandbox/tree/johanvos-headless

This addresses https://bugs.openjdk.org/browse/JDK-8324941 where I suggest
a POC for a Headless platform.

There are a number of usecases for this, including:
1. applications that require JavaFX rendering without presenting this to a
window (and instead send it to a printer for example)
2. running tests without requiring a window manager.

Regarding the second usecase, we already did some basic experiments using a
modified version of TestFX where instead of the Monocle Headless
subplatform, the POC Headless platform is used.

By using a first-class Headless glass platform instead of a Monocle
subplatform, it should be easier to use by developers.
The monocle code contains very platform/os specific parts, which often
don't make sense outside the target platform. This is very valuable, but it
is also a very different usecase than a headless platform and it requires a
much more complex build procedure.

I added an initial, limited HeadlessRobot to do some basic tests. That code
is mainly taken from the existing Monocle implementation, but I want to be
careful to avoid anything that is not applicable to the headless scenarios.

- Johan
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