downstream bug from ubuntu:
started happening for more people when the default fallback session started using llvmpipe. Applies to 8.0.x and 9.0.
It's worked around for now ("Check that pixmaps which aren't managed by us actually exist before binding") but a proper fix should be added to mesa.
IIUC the compiz-core patch
the fix consisted of not calling glXBindTexImageEXT when
XGetGeometry (dpy, pixmap, ...)
https://www.opengl.org/registry/specs/EXT/texture_from_pixmap.txt does say that
"Currently, only pixmaps can be bound to textures. If <draw> is not a
valid GLXPixmap, then a GLXBadPixmap error is generated."
so indeed this should be handled better.
This technically is a bug in st/dri though.