add detect for background-blend-mode

fixes #1388

Unfortunatly, "[...] in some browsers, mix-blend-mode is implemented; the property is valid, the automated tests pass, but no blending actually takes place" - http://blogs.adobe.com/webplatform/2013/09/12/browser-support-matrix-for-css-blending/

background-blend-mode doesn't seem to have that issue, however