Single buffered clients (LP: #1194333)

[Christopher James Halse Rogers]

We've got a bug filed¹ documenting that we don't support 
single-buffered clients. I'd like to mark this as WontFix.

The reasoning being: single buffered clients cannot reliably render 
without glitches, EGL allows us to not support frontbuffer rendering, 
and our current pseudo-support for single buffering results in the 
BufferBundle being more complex than it needs to be.

Since it doesn't work now, and cannot work well, I suggest we annotate 
the bug as such.

¹: https://bugs.launchpad.net/mir/+bug/1194333