When property::surface is emitted, the drawable didn't know its geometry yet,
which had weird side effects. Fix this by changing the C API a little. The
function drawable_set_surface() now no longer allows a NULL surface as its
argument.
The required changes for the titlebar code also means that we no longer throw
away the double-buffering surface when a client is moved.
Signed-off-by: Uli Schlachter <psychon@znc.in>