When a with bg is selected, the cell's regular background will be blended
with 50% opacity selectionBackground in order to retain the bg color info
as well as getting the color to change and getting decent contrast.
This commit also fixes a terrible, nasty bug where the standalone shared
renderer tests were causing the terminal to open multiple times and not
get the WebGL addon activated the second time. So some tests were actually
testing the DOM renderer :o
Fixes#4918
Daniel Imms
