RFR: 8236651: Simplify and update glass gtk backend

Wed Jan 13 13:00:52 UTC 2021

On Fri, 12 Jun 2020 12:49:42 GMT, Pankaj Bansal <pbansal at openjdk.org> wrote:

> After the latest commit on June 10, this is not building for me on my Ubuntu 18.04. I am attaching the build log for reference.
> [build.log](https://github.com/openjdk/jfx/files/4770864/build.log)

It's now fixed. I had used two compilation parameters to limit Gtk on 3.8 (so it would generate error if any symbol > 3.8 were used). But that does not seem to work on 18.04, so I removed it.

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PR: https://git.openjdk.java.net/jfx/pull/77