please add a texture scan lookup callback

[quiret]

In original Criterion RW, there is a function to set the current global texture lookup callback. If you set this callback then everytime rw::Texture::read is called your callback is called instead. This is really handy because you could do a lot more than just get the textures from the current TXD... you could even link look inside more TXDs than just the current TXD!

Please add such a functionality. It has been added since RW 3.5.