docs: Add initialization example for GtkGLArea
docs: Add initialization example for GtkGLArea

Show how to safely check for errors when initializing the OpenGL
pipeline.

davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,jigpu/gtk,Adamovskiy/gtk,davidgumberg/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,chergert/gtk,Adamovskiy/gtk,jadahl/gtk,ahodesuka/gtk,davidgumberg/gtk,jigpu/gtk,Adamovskiy/gtk,chergert/gtk,ahodesuka/gtk,davidgumberg/gtk,chergert/gtk,ahodesuka/gtk,alexlarsson/gtk,jigpu/gtk,ahodesuka/gtk,chergert/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,grubersjoe/adwaita,chergert/gtk,ahodesuka/gtk,jigpu/gtk,ahodesuka/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk