Add support to only create sprites with dividable width and height

[peterrietzler]

If the generated sprite would have an odd width or height, then add transparent pixels to width or height, so that images can always be downscaled properly without losing "parts of a pixel" (e.g. add 1 pixel to have even width, height if 2x and 1x is targeted)

E.g. when a resulting retina (2x) sprite has a size of 100x99, then the size of the standard (1x) sprite would be 50x49.5. The generated CSS media query sets the background-size property to either 50x50 or 50x49 in this case - and you will e.g. experience "moving icons" when you use it for hover/active effects.

[erickreutz]

+1

[jorgebastida]

@peterrietzler I think the best solution would be to make glue show a warning if your input image sizes are not divisible by any of the ratios. Thoughts?