First pass at implementing the PulseEcho shader.

This is based on the Echo shader found here:
http://www.seethroughskin.com/blog/?p=1716.
Move state management out of Metagame and into PingReceiver; add a detection duration setting.
Set the PulseEcho shader queue to Overlay to get it rendering above the mask planes.
Get the pulse material updating via PingReceiver.

Still needs a bit of work.
Check in assets related to the PulseEcho shader and material.