Closes #135: More intuitive implementation of background mute
Closes #135: More intuitive implementation of background mute

Added a locked version of `toggleLockedLayerActivation`. Sets opacity
to 1 whenever background is turned off and remembers the last opacity
so canvas returns to previous state when background is turned back on.

DDMAL/Pixel.js,DDMAL/Pixel.js,DDMAL/Pixel.js