GPUImageNormalBlendFilter filter fix for premultiplied alpha

The image input is premultiplied alpha, as that is what is generally produced out of GPUImagePicture. This filter assumed non-premultiplied alpha, which resulted in darkening when blending images with alpha. I have modified the formula to use the premultiplied alpha formula for compositing from http://en.wikipedia.org/wiki/Alpha_compositing

It also has the advantage that it's a simpler formula!

This follows on from my erroneous screen blend mode pull request that I closed earlier. I went on a wild goose chase when I thought it was the screen blend mode that was broken rather than normal. Hopefully I'm not chasing geese again here!