RFR: 8340378: [XWayland] FXCanvas apps and tests crash on Ubuntu 24.04

[Jayathirth D V]

As mentioned in recent PR : https://github.com/openjdk/jfx/pull/1890#issue-3401783091, we need to set environment variable GDK_BACKEND=x11 for running HelloFXCanvas also.

There is no correct way of setting environment variable through Java code/Netbeans project files. So we are capturing this additional step for Linux in README.txt.

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Commit messages:
 - 8340378: [XWayland] FXCanvas apps and tests crash on Ubuntu 24.04

Changes: https://git.openjdk.org/jfx/pull/1898/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1898&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340378
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1898.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1898/head:pull/1898

PR: https://git.openjdk.org/jfx/pull/1898