LIBGL_ALWAYS_SOFTWARE=1 with stock linux drivers gives black window
When the environment variable LIBGL_ALWAYS_SOFTWARE is set on linux to force the use of llvmpipe instead of hardware rendering, the resulting window is black.
Bisection shows that this was caused by !6965 (merged), which changed framebuffer handling.
To reproduce, run a VTK test as follows on Linux with stock (not proprietary) drivers:
LIBGL_ALWAYS_SOFTWARE=1 bin/vtkInteractionWidgetsCxxTests TestCameraWidget
I did the testing on Ubuntu 20.04 with amdgpu and nouveau drivers
OpenGL vendor string: Mesa/X.org OpenGL renderer string: llvmpipe (LLVM 12.0.0, 256 bits) OpenGL core profile version string: 4.5 (Core Profile) Mesa 21.0.3
Note that the Qt tests render correctly, while tests that use vtkXOpenGLRenderWindow produce a black window.