Alternative images

Adds optional alternative format support to picturefill (JPEG-XR, JPEG-2000, WEBP, SVG and APNG).  This option will only work if picturefill is built by setting the support-types grunt task correctly when building picturefill. For example, to support all these formats, build picturefill with this grunt task:

grunt support-types:svg:webp:jxr:jp2:apng

If you wish to remove support for any of these formats, take them out of support-types task. If you wish picturefill to support none of these alternative formats, you can leave out the support-types task out of the grunt build altogether.