half done ffi pic module

The draw method for this ffi implementation is not done yet.

So far, what I get is, compared to the original C implementation, my ffi implementation takes longer to complete all the tests.

I sent out this PR because it's my first time playing with luajit ffi and I might have done something stupid. I am hoping if someone can point it out ;p

HINT: the nested for loop where I convert rgb to gray scale might be the cause. But I don't know how to optimize it :( The draw method is probably going to make it worse since it also involves nested for loop.