Stencil PMA fix
In the future...
We should implement the Stencil PMA fix. We may not want to apply it in general - apparently it can hurt in some cases. I think Windows maintains a whitelist of apps that it helps.
`pma-stencil' of ~kwg/mesa has old i965 code for this. I stalled out when discovering that the documented PIPE_CONTROL rules don't work at all. Jason did this in Vulkan a while ago, using the same stalls as for the Gen8 depth PMA.
Alex at Feral noted that the stencil PMA improved performance by 13% in one of their Vulkan benchmarks, so it's likely worth porting to GL.
https://lists.freedesktop.org/archives/mesa-dev/2018-January/181204.html