Trying to access struct internal variable

*Moved issue from https://github.com/raysan5/raylib/issues/204* When trying to use `GenTextureMipmaps()` in example [`text_ttf_loading.lua`](https://github.com/raysan5/raylib/blob/master/examples/text_ttf_loading.lua), it throws the following error: `ERROR: Lua Error: text_ttf_loading.lua:29: attempt to index a SpriteFont value (local 'font')` It seems it can not access the texture variable inside the SpriteFont struct.