Enforce premultiplied alpha in GPUImagePicture

In keeping with what I believe is the correct understanding, that GPUImage expects images to be in premultiplied alpha, I have modified GPUImagePicture so that if the input is not premultiplied (or no alpha) that it redraws it. This is further to the image data compatibility pull request that was accepted earlier - we no longer allow unpremultiplied alpha to go through, we redraw.