* TODO
** DONE [#A] observer slots
** DONE [#A] cached slots
** DONE [#A] reloadable assets
** DONE [#B] 2d render culling
** DONE [#A] FPS display
** DONE [#A] In-engine REPL
** TODO [#C] global minor modes
** TODO [#C] graphical widgets
** TODO [#C] node-3d