@Imperf3kt, thank you for pointing me on the right track. I bought another memory card and put Raspbian (Stretch) on it as a positive control. Using raspi-config G7-Advanced A7-OpenGL, I selected 2-Fake KMS. I couldn't find a .deb package for GLMark2, but I did find and install globs-0.2.0~svn50-4 ....