Support render performance testing

[tom-schultz]

I am using the TPS demo to test Godot render performance for some upcoming 4.x render performance improvements. To test those improvements I'm adding features to the TPS demo to test render performance. This seems like something that could be useful to other folks as well.

My suggested enhancements:

• Support a "render test" mode that stresses the GPU
  • Use the existing static 3d camera in front of the reactor core is already in a good position to do so
  • Add a directional light with shadows on for even more graphical intensity (render test mode only)
  • Disable player spawning
• Add a setting to enable render test option that's saved to Settings
• Add command line options to enable render test mode and exit after X duration

I am developing all of these features on the tagged PR.