Re-add namespace for GUI render event

[jennuine]

🦟 Bug fix

Summary

A required namespace was accidentally removed in #1635 , this caused the assumption that the render event was a ignition::gazebo::gui event when it needed to be an ignition::gui event.

To test:

ign gazebo shapes.sdf

then use the Screenshot plugin.

Before PR, nothing happened when clicking the screenshot icon (no popup stating where image was saved and no image saved to ~/.ignition/gui/pictures).

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[iche033]

there are other places in ign-gazebo3 that uses gui::events::Render. Do they need to be updated as well? e.g. https:/gazebosim/gz-sim/blob/ign-gazebo3/src/gui/plugins/grid_config/GridConfig.cc#L90

[jennuine]

Nope, the bug lived in the sender. Anything listening to the render event doesn't need updated since both a ignition::gui::events::Render and ignition::gazebo::gui::events::Render are being sent: https:/gazebosim/gz-sim/blob/ign-gazebo3/include/ignition/gazebo/gui/GuiEvents.hh

Since both events are being broadcasted, plugins listening to either render event will be caught now (doesn't matter which one but preferably ignition::gui::events::Render since ignition::gazebo::gui::events::Render was removed in fortress)