RFR: 8236651: Simplify and update glass gtk backend

[Kevin Rushforth]

On Tue, 9 Jun 2020 23:12:15 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> I ran the systemTests on Ubuntu 18.04 using
>> gradle -PEXTRA_TEST_ARGS='-Djavafx.gtk.experimental=true' -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:cleanTest
>> :systemTests:test. Following are the results  <img width="969" alt="Results_18 04"
>> src="https://user-images.githubusercontent.com/6153953/83944842-359d1f00-a824-11ea-964b-f7d9664af9f3.png">
>
> In case it is useful to those reviewing the code, I have created a [WIP pull
> request](https://github.com/kevinrushforth/jfx/pull/1) in my repo solely for the purpose of looking at the diffs
> between the existing gtk native code and the experimental GTK native code from *this* pull request.

I am running a full test using GTK 3 on Ubuntu 20.04 and will publish results. I will later do the same for Oracle
Linux 7.7.

One thing to note is that this new GTK pipeline doesn't run on Ubuntu 16.04. I get the following running any program:

$ java -Djavafx.gtk.experimental=true -Djdk.gtk.verbose=true HelloRectangle
checking GTK version 3
trying GTK library libgtk-3.so.0
using GTK library version 3 set libgtk-3.so.0
Glass GTK library to load is glassgtk3_exp
loaded gdk_x11_display_set_window_scale
java: symbol lookup error: /localhome/kcr/javafx/jfx-tmp/jfx/rt/build/sdk/lib/libglassgtk3_exp.so: undefined symbol:
gdk_display_get_n_monitors

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