Previews for shared buffer PR

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