Previews for shared buffer PR

[Johan Vos]

The PR discussed in https://github.com/javafxports/openjdk-jfx/pull/472,
addressing https://bugs.openjdk.java.net/browse/JDK-8167148 provides a very
much wanted feature. It is important that things are done in the right way
so that the code can be maintained in the long-term future.
Therefore, feedback on this PR is extremely important before we can
consider merging it. Once this PR is merged, there is no easy way back. It
is possible to add more functionality, hence my preference is to only
implement the functionality that is safe and stable, while allowing other
functionality to be added later or by third-party extensions. (e.g.
(avoiding) copying from/to GPU)

To make it easier to give feedback, we've build early access versions of
SDK's including this PR. Note that the PR is not merged, hence not
available in the regular EA downloads!

If you want to give it a try, download the SDK's from the URL's below.
There is a test in tests/manual/graphics/PixelBufferPerformanceTest (
https://github.com/arapte/openjdk-jfx/blob/JDK-8167148-NIO-ByteBuffer/tests/manual/graphics/PixelBufferPerformanceTest.java)
that should get you started.

https://download2.gluonhq.com/openjfx/forks/pixelbuffer/openjfx-13-pixelbuffer-ea+9_linux-x64_bin-jmods.zip
https://download2.gluonhq.com/openjfx/forks/pixelbuffer/openjfx-13-pixelbuffer-ea+9_linux-x64_bin-sdk.zip
https://download2.gluonhq.com/openjfx/forks/pixelbuffer/openjfx-13-pixelbuffer-ea+9_osx-x64_bin-jmods.zip
https://download2.gluonhq.com/openjfx/forks/pixelbuffer/openjfx-13-pixelbuffer-ea+9_osx-x64_bin-sdk.zip
https://download2.gluonhq.com/openjfx/forks/pixelbuffer/openjfx-13-pixelbuffer-ea+9_windows-x64_bin-jmods.zip
https://download2.gluonhq.com/openjfx/forks/pixelbuffer/openjfx-13-pixelbuffer-ea+9_windows-x64_bin-sdk.zip

- Johan