For correct lookups, without our texture replacement actually supporting
volume textures, we need to use this mechanism here too.
The game actually uses two mipmaps, but they're identical and point to
the same memory, so we treat them as a regular 2D texture instead for
purposes of both texturing and replacement. This is presumably legacy
from the initial Japanese version that needs to use multiple texture
layers. Similarly it does in in pairs.
This does actually not fully fix texture replacement for the Japanese
version, unfortunately. For that we need more proper support for these
weird textures in the texture replacement code - when I refactored it
before for more natural handling of regular mipmapping, this kinda got
lost.
Fixes#17268, but I feel we could possibly get rid of the bool entirely
since we have maxlevel. Only GL and D3D even looks at this flag.
Oh well, that's for later.
Fixes#17182
Not exactly sure what behavior we really want, but I think this one is
OK, and at least more similar to the old one. Now we save
already-replaced textures if the named replacement texture is missing, and there
isn't already a hash-named one in new or the "root".
