AppImage build does not support OpenGL mode

As originally discussed at !4133 (comment 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.