Mesa 18.1.6 breaks plasma on BPO

[Julien Aubin <julien.aubin@gmail.com>]

Hi

I've seen a weird issue with the latest Mesa update w/ KDE : the
Plasma panel appears to be broken. In the best case transparency is
missing, in the works the bottom panel (the one with the KDE menu)
becomes dark grey but could not reproduce it on all the machines.

My config :
- Debian Stretch amd64 w/ all updates
- Mesa 18.1.6
- Direct rendering / OpenGL 3.1
- Theme : breeze w/ Kwin default settings)
- Issue reproducible with both Intel and NVidia/nouveau drivers.

The issue did not appear with Mesa 17.xx and does not appear with
Nvidia blob. As long as direct rendering is enabled the issue appears
(ie w/ OpenGL 2.0 it appears as well). Disabling transparency does
nothing.

The only workaround is to use XRender.

Could you please fix the issue as black text over dark grey is REALLY annoying ?

Thanks.