add the ability for the preview to use the textures from a customgui texture pack please and thanku luv u bye

Hi, resource pack customisation has been available for a while (the begging even). To override the texture, a pack needs to place a PNG image at assets/shulkerboxtooltip/texture/gui/sprites/shulker_box_tooltip.png and assets/shulkerboxtooltip/texture/gui/sprites/shulker_box_tooltip.meta if the texture doesn't have the same size as the original.