Resizing, Unity8, BufferStreams oh my!

[raof at ubuntu.com]

While implementing multi-BufferStream surfaces in qtmir I've realised 
that it's extremely difficult for the shell to do anything but 
composite a client's buffers exactly as-is during resizing. However, we 
sometimes want to be able to stretch (or partially render, in the case 
of rotation) client surfaces.

At the same time, we'll need some way of updating all a client's 
BufferStreams atomically.

I've put up a Google doc with the problem as I see it, and a proposed 
approach.

https://docs.google.com/document/d/1Se4kbUHZNbgB9e3lrr3EQnVnQgSgFThyiBJ0MDgkZ0c