Issues showed up after #114:

• Catastrophic slowness: Loads of issues resulting from use of VkFFT #129 (comment)
• Missing global barrier in some cases (FYI @isuruf this is not #564, it's a problem I haven't talked about yet)
  • This was a un-upgraded loopy
• Crashes, e.g. https://github.com/inducer/pytential/runs/7625741382?check_suite_focus=true
  • pocl crash in macos only with out of bounds access pocl/pocl#1084
  • Add a fallback for FFT when VkFFT is not found or broken #130
• VkFFT errors on Mac: Loads of issues resulting from use of VkFFT #129 (comment)
• Intermittent VkFFT build failures: https://gitlab.tiker.net/inducer/sumpy/-/jobs/443459

cc @isuruf