AppImage build does not support OpenGL mode
As originally discussed at https://gitlab.com/inkscape/inkscape/-/merge_requests/4133#note_866924284, OpenGL support is unusable in the AppImage build. When enabled, it falls back to Cairo with the following error message:
Failed to create OpenGL context: No available configurations for the given pixel format
Steps to reproduce, on Linux (X11 or Wayland):
* Open Inkscape.
* Enable Preferences / Rendering / Request OpenGL.
* Observe the terminal output.
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