Added color-coding to deferredFragShader: magenta=invalid TOC magic,
green=TOC index is zero (unset).
Added magic to TextureTOCWindow.
Texture now sets magic in TOCWindow.
Refactored BackdropSystem to use Sprite2D. Fixed non-rendering bug
(probably due to backface culling blocking out wrong vertex order in
older model approach)
Added activation of BackdropSystem in Game's startup sequence.
Bugfix: Mission did not remove persistent message soon enough, resulting
in it hanging over into gameplay when overworldsystem is activated.