ImageService - use "factors" for pixel-perfect sizes

Get full control over auto-generated image sizes.

By default the Image Service will calculate sizes based on the factor. Example:

• Screen size is 1200px
• Factor is 0.5
• Image would be assumed to show at 600px

But in reality, there is probably a gutter (space between images), so maybe the image should only be 575px wide. 

When this feature is enabled, the ImageService will take into consideration all the special settings configured by the designer, to use pixel-perfect sizes for each factor.