The depth member of FbWindow was abused to store the maximum depth
but that gets overridden with geometry changes of the root window
(screen layout changes) so we store and read the value explicitly while
::depth() maintains the actual depth of the root window
The result of this is that frames for ARGB windows were created with a
wrong depth and failed to reparent the client window.
BUG: 1102
BUG: 1058
fluxbox now properly displays windows that require ARGB visuals when
an external compositor is running. This was done by creating the
container window with the correct visual and colormap when needed.
Closes#2874629
