Hughes Ross has started to work on a PR for this
(https://github.com/minetest/minetest/pull/10140)
but it doesn't look like it will be ready until 5.4 at
least, and we will support pre-5.4 versions for
some time yet. The ability for TP artists to
override special_tiles is necessary to allow things
like flowing liquid textures to be customized in
texture packs.
Add a hack for these, similar to how we did the
inventory/wield image hacks.
Note that we are also forcing overrides for special
tiles, since there may be cases where we reuse a
base image without modifiers but TP artists may
still want an independent override.
78d321c24f