Code review comment for lp://staging/~vanvugt/mir/parenting-server-api

OK, I tried making set_parent a one-parameter function. It got really ugly. I then had to document that query(mir_surface_attrib_parent) is non-functional, also lost parent-changed event support, and the resulting hacks required to make tests work again were too ugly. Because "parent" then became effectively not a surface attribute any more.

set_parent with two parameters is certainly cleaner, and more functional.

Alternatively, we could clean it up by adding an id() to scene::Surface, but I think that's unnecessary, more complex, and would be equally easy to abuse. The current proposal seems best.