modular_aphelion/modules/custom_sprites/code/base_marking_overlays.dm

/proc/body_marking_sheet_size	Returns the size of a body marking sheet as "[width]x[height]", read once per sheet. moth_markings.dmi is 45x34, every other sheet 32x32. Null for an icon built at runtime, whose text names no file ("/icon" for every one), so a content key could not tell two apart.
/proc/body_marking_sheets_blend	Returns whether markings on two sheets can blend into one icon: sheets of one size, and never an icon built at runtime.
/proc/merge_body_marking_run	Draws a run of markings, or of glows, with its first appearance as drawn into the output list, a dropped limb's image of it included: that appearance takes the run's blended icon and keeps its own colour, alpha, layer, plane, flags and facing, so it draws what the run's appearances drew one on another.