Support Generating ImageCache Images

Currently using ImageCache and Image Resize Filter has some caveats. Image Resize Filter can resize ImageCache images just fine, as long as the ImageCache derivative is generated first. In the situation where the ImageCache image does not exist, Image Resize Filter will fail. It'd be optimal to generate the ImageCache derivative if necessary, then continue letting Image Resize Filter resize that.

This patch adds support for ImageCache in this way.