Make the background of plain web content transparent.

I made it so this only takes effect if you are using plain content. I guess the assumption is that if you are using the webvfx javascript extension, you have more control over the placement of the image, especially z-order. Also, it neatly avoids regressive behavior on any existing html webvfx filters and transitions.

This is already useful with the MLT filter as-is and allows HTML with no
explicit background color and no WebVfx JavaScript extensions to simply
overlay the MLT video.