layout: Try to allocate before getting size of tracked actors

Because we're about to get_transformed_{position,size} of each, which will return NaNs if not yet allocated. Those NaNs were finding their way into the workspace strut definitions on startup and not getting corrected until after the startup animation completed. This meant any extensions depending on the workareas-changed signal were getting an incorrect workarea (the whole workspace) and so were rendered out of place during the login animation. Now they're not.

Fixes: mutter#1627 (again) and https://launchpad.net/bugs/1919979