Code review comment for lp://staging/~raof/qtmir/multi-bufferstream-support

I don't know Mir enough to say whether letting clients paint outside of their bounding rect is good or not, but if so, the compositor would sometimes have to switch on clipping based on UI constraints, like for instance in spread mode. (in this case, which seems likeley, I guess we'll have to expose a property like "clipRenderables" on MirSurfaceItem that makes clipping more efficient than the QtQuick default technique, by simply setting vertex positions and texcoords of the renderable textured quads accordingly.)