Summary
-------

Provide an implementation system property which controls the platform API used by java.awt.Robot on Linux to capture screen pixels.

Problem
-------

Modern Linux desktops now enable the Wayland display server by default.
X11 applications like JDK then run on the compatibility layer, XWayland. 

This has some limitations, one of which is that you cannot take a screenshot with the currently used X11 API,
and this is one of the reasons JDK is not supported at all running on the Wayland server, since
the Robot API functionality is severely degraded and even though this is now allowed by specification
[see the CSR JDK-8308012: Relax the java.awt.Robot specification],
it is a practical limitation for some applications and is a blocker for automated testing.

Solution
--------

Use newer platform API for taking screenshots on Linux desktops which works both on Wayland and X11.
This will be enabled by default if Wayland is detected (but note this mode is still unsupported) and
can be explicitly enabled on X.org/X11 servers via setting a system property. 

The platform API to be used is the [org.freedesktop.portal.ScreenCast](https://flatpak.github.io/xdg-desktop-portal/#gdbus-org.freedesktop.portal.ScreenCast) DBUS API.
This will be an important step to later supporting JDK on Wayland in X11 compatibility mode.


Specification
-------------

**New system properties for Linux only **

`awt.robot.screenshotMethod` - `dbusScreencast` by default for Wayland session, `x11` by default for X11 session, and for all other values except `dbusScreencast`.


Note 
<ol>
<li>setting this to `x11` in a Wayland session means screen capture will always fail, as it does in earlier JDKs</li>
<li>setting this to `dbusScreencast` in an X.org session should work but is only recommended for JDK debugging purposes</li>
<li>The usage and meaning of the already existing `awt.robot.gtk` System property is unchanged and is only valid for `x11` and has no effect for `dbusScreencast`.</li>
</ol>


`awt.robot.screenshotDebug` - `false` by default, if `true` prints debug information related if any is available.