[Dev-luatex] Allow usage of img module from texlua

Hi Luigi, I'm currently reimplementing the "extractbb" program in Lua, but for this to work properly I need access to the "img" module from Lua-only mode. Right now I'm using a questionable hack to make this work https://github.com/gucci-on-fleek/extractbb/blob/v0.0.7/source/extractbb-scr... but I'd prefer it if I could do this without using the FFI module. I've attached a (rather trivial) patch to allow this. Thanks, -- Max