Draw opaque entities atop robots

[byorgey]

I wonder about making opaque also imply that the entity should be drawn with a higher priority than robots?

I would be in favor of having the opaque property affect the display. It is consistent with the physical interpretation w.r.t. the scout command.

Originally posted by @byorgey and @kostmo in #2245 (comment)

This might require modifying Display a bit to add an "opaque" flag; or perhaps it could be as simple as mapping opaque to a specific numeric display priority which is higher than the default robot priority.