Background image is part of the scene (always behind all elements).
Stretch+KeepAspect: fit inside stage (whole image visible). Stretch+no aspect: fill stage.
If Stretch is off: natural size centered, clipped.
Used by live SVG display and all SVG/PNG/PDF/HTML exports. Number = y offset em. Object supports {"y":0,"x":0,"scale":1}. Example: {"TP-Nasin-Nanpa-Font":{"glyph":{"scale":1,"y":0.02},"sitelen":0.02},"default":0}