Mesa Clover: 15% ~ 20% slower between 2018 and 2021

• Clover: 33028
  2021-12-04, Mesa 20.0.4¹, http://luxmark.info/node/9180
  Ubuntu 21.10, Linux 5.13.0
• Clover: 36155
  2020-06-22, Mesa 20.0.8, http://luxmark.info/node/8102
  Ubuntu 20.04, Linux 5.4.0
• Clover: 42600
  2018-11-24, Mesa 18.2.4, http://luxmark.info/node/5855
  Ubuntu 18.10, Linux 4.18.0

¹ Older Mesa from Ubuntu 20.04, the one shipped in Ubuntu 21.10 is broken (see #16 (closed)) and current builds from Oibaf PPA doesn't work either.