Fixed detect for subsampling for 0.5x and 0.25x

I have notice that iPhone subsample as 0.5x and iPad as 0.25x. To future proof it I made a check that creates two canvases and checks if it pixels from the image "covers" the canvas or not. But in this case I make it offset a half respectively a quarter of the image width to check how much the canvas subsamples.

I have been tested this both no iPhone and iPad. And it works.