Two reports of this so far on Gentoo when using Mesa 17.0.0 RC2.

See a screenshot here:
https://bugs.gentoo.org/attachment.cgi?id=461558

I am reporting this bug upstream (i.e., here) at the request of Gentoo dev Matt
Turner.

Here is the Gentoo bug report:
https://bugs.gentoo.org/show_bug.cgi?id=607368

The problem disappears upon downgrading to Mesa 13.0.3.

Mateusz Lenik, the reporter of the bug on Gentoo's tracker, is using a Polaris
10 card, and I am using a Tobago (Bonaire Pro) card. We are both using the
"radeonsi" VDPAU driver.