Picture load callback and size access

Please add this functionality, so it is easier to access the width/height (and therefore scaleRatio) of the underlying original image, which is useful for lots of View calculation and other filters.
Furthermore, I added an OnImageLoadedListener. A usecase would be to have a GPUImageTwoInputFilter, which should receive the rendered bitmap of another filter as the second texture  (GPUImageTwoInputFilter#setBitmap(Bitmap bitmap))