Add support for adjustable sprite image spacing

Motivation

When using the browser's zooming feature, interpolation of sprite images can cause unpleasant effects on the sprite image boundaries. A possible solution to this problem would be introducing non-zero spacing between sprite images, as suggested on the discussion group.

Implementation

Incomplete specification

The implementation will introduce a global, sprite and sprite image-level sprite-spacing property for controlling spacing between individual images.

Discussion

For the time being, it should be enough to add spacing only along the layout direction of sprites. However, when optimized sprite layout is implemented (SMARTSPRITES-64), spacing will have to be added on all sides of individual images, except on the sprite image borders.