GPUImageNormalBlendFilter filter fix for premultiplied alpha

The input is assumed to be premultiplied alpha. I believe this filter assumed non-premultiplied, which resulted in darkening where there shouldn't be.
In GPUImagePicture, if the source does not have premultiplied alpha (or no alpha) then redraw so we consistently have premultiplied alpha in our input images
Merge remote-tracking branch 'origin/normal-blend-mode' into develop
Merge remote-tracking branch 'origin/picture-premultiplied-alpha' into develop