Some users may use filtering but not have the "clean transparent"
filter enabled in the Minetest engine config. Bake this filter
into all images to (1) avoid the consequences of this (i.e. black
halos), and (2) possibly speed up startup time (the filter will
converge immediately).
Use the upstream coal textures from the fire mod, since that's
already a dependency, and we can make sure our textures are
consistent with the upstream ones. Make the burning torch tip
look embery and not just coaly.
