Returns NaN when width and height are not divisible by window size

[georeith]

Comparing two images I get NaN as the returned MS-SSIM value unless the width and height of them is divisible by the window size.

However SSIM often returns correctly. Inspecting it appears one of the SSIM comparisons always returns NaN and by multiplying them together the overall becomes NaN.

I have also noticed the test browser incorrectly specifies image width, where it uses the default width instead of canvas.width:

ctx.drawImage(img, 0, 0, width, canvas.height);

image-ms-ssim-js/test/browser_test.html

Line 187 in c47ada6

ctx.drawImage(img, 0, 0, width, canvas.height);