[Xcb] xcb support for mesa

[Christoph Pfister]

Hi,

Are there plans to add "full" xcb support to mesa, so that you can use
for example xcb connections and don't need Display* anymore?
caro made me aware that 7.0 contains some xcb code, but it's only used
internally and nothing xcb related is exposed to the api.

Thanks,

Christoph