Use percent orig width/height for down scaling

[benlubas]

This change better respects the users configured max width and height values when rendering an image. Addressing #19

higher level overview of how

When down-scaling images, we crush the image to the max height and width, and then attempt to restore the aspect ratio. When restoring the aspect ration, we now take the percentage of the original dimension (either width or height) that's smaller, keep that smaller value, and scale the other to match.

This should always be more accurate than what we were previously doing, which was just keeping the larger value and scaling the smaller value to match.

[3rd]

This looks great, thank you!

Just noticed that the math.ceil there might make the images look worse than by using math.floor when setting max sizes.

Both will have an error margin, but floor seems closer to the original...
Think we should switch to floor?

original:

ceil + no max:

floor + no max:

ceil + max_width 5:

floor + max_width 5: