Ninja-night vision now works.

A ninja now uses proper glasses instead of using its own hack.
Fixed a bug where ninja-night vision would be overriden unless one also wore additional glasses in the glasses-inventory slot.
Adds overlays to thermals and mesons as well. Slightly reduces the intensity of the night vision overlay.

Based on #6311. Fairly big changes though hence dev instead of dev-freeze/master.
For those curious, this is where ninja-night vision was overriden and where the suggested workaround comes from.
Pseudo-code:
````
if(wearing_ninja_mask)
  apply-night-vision-if-active

if(wearing_glasses)
  apply-glass-vision
else if(!seer)
  remove-all-glass-visions
````