Physics breaks at high altitude at high framerate

[blast007]

High altitude combined with a high framerate really messes with the game physics. It becomes difficult to even drive on a flat surface since your tank keeps thinking it isn't on the ground. Gravity also gets a bit broken so you either jump higher or seem to get stuck in the air when the Z velocity nears zero. At higher altitude (for example, 1000) it even seemed like the jump height issue got worse than in the example world below. Starting at around 600 FPS, you tend to float across flat objects for several seconds as though you were in the air falling.

Here is a basic world example that can trigger this:

box
size 800 800 800
end

On my Linux system, I was able to trigger the issue very visibility starting at 567 FPS (/localset fpsLimit 567 with Energy Saver set to FPS Limit). At 566 and 565 there were still some instances where the tank would "land" while driving on a flat surface. I'm sure I'd have to back the FPS off a bit from there to keep it stable. I know we've had other physics issues with high FPS, such as the tank bouncing off objects starting at around 200 FPS.

bzflag_high_physics.mp4

We can't really fix the physics in 2.4 since that would break compatibility, but perhaps we should always have a max FPS limit of 200 regardless of what the energy saver option was set to? Disabled for debug builds but enabled for release builds.