Improve comments
New "Add Film Grain" tool

I did a fair amount of research on this tool before implementing it.
It's astonishing how many parameters some software provides - there are
whole software packages based around adding film grain to images, and
some even go so far as to include a huge set of textures from famous
cameras!

PhotoDemon's approach is obviously much simpler.  Only luminance noise
is added, and two parameters are tweakable:

1) Strength (e.g., how far to throw pixel luminance)
2) Softness (at 0, the noise is clearly digital and fake; higher values
smooth the noise before applying it, making the effect more subtle and
realistic)

Performance is quite good, on account of only luminance being touched.