Export a sprite sheet keeping the background opaque for indexed sprites

Export Opaque Sprite Sheet.lua

local spr = app.activeSprite
if not spr then return print("No active sprite") end
app.command.ExportSpriteSheet()
if app.activeSprite ~= spr and
   spr.backgroundLayer and
   spr.colorMode == ColorMode.INDEXED then
  local oldColor = app.bgColor
  app.bgColor = Color(spr.transparentColor)
  app.command.BackgroundFromLayer()
  app.bgColor = oldColor

  app.command.SaveFile()
  app.command.CloseFile()
end