Display problems in Linux agents

[Crola1702]

Some Linux gazebo builds are showing display problems:

• https://build.osrfoundation.org/job/ignition_gazebo-ci-pr_any-ubuntu_auto-amd64/10649/
• https://build.osrfoundation.org/job/ignition_gazebo-ci-gz-sim7-focal-amd64/136/
• https://build.osrfoundation.org/job/ignition_rendering-ci-main-focal-amd64/70/

Log output:

-- Looking for display capabilities
--  ! valid display not found
--  ! valid dri display not found (no glxinfo or pyopengl)

then:

65: qt.qpa.xcb: could not connect to display unix::0

or

11: [Err] [OgreRenderEngine.cc:358] Unable to open display: unix::0

Depending on the job.

Failing test regressions because of this problem:

• INTEGRATION_follow_actor_system.test_ran
• INTEGRATION_model_photo_shoot_default_joints.test_ran
• INTEGRATION_model_photo_shoot_random_joints.test_ran
• INTEGRATION_user_commands.test_ran
• UNIT_Gui_TEST.test_ran
• WorldFiles/GazeboDeathTest.CleanExit/0
• WorldFiles/GazeboDeathTest.CleanExit/1
• UNIT_JointPositionController_TEST.test_ran
• UNIT_Plot3D_TEST.test_ran

---

I have as the main culprit: #916

I ran the same job with display detection PR reverted (#978):

• https://build.osrfoundation.org/job/ignition_gazebo-ci-gz-sim7-focal-amd64/138/

Log output for this build shows that it is detecting display:

-- Looking for display capabilities
--  + found a display available ($DISPLAY is set)
--  + found a valid dri display (glxinfo)