glamor misrendering

Some extra color is rendered on some UI elements when running Gtk+ program inside glamor-accelerated X.

Here's a trace recorded with Xephyr, which can be replayed with GLX on a normal X server:

https://mega.nz/#!mspHSKiD!Eau-x7SHjUZjakptnP-U4uohd_EMgkNeRWBzhi-Bpj8