Physics movement is not interpolated.

[drwhut]

Describe the bug
If the refresh rate of the monitor is higher than the physics framerate (default is 60Hz), then physically-driven objects visibly stutter due to the object's movement not being interpolated in-between physics frames.

To Reproduce
Steps to reproduce the behavior:

1. Open the game via the Godot editor.
2. Lower the physics FPS in the project settings to around half the refresh rate of the monitor.
3. Launch the game, and enter a singleplayer instance.
4. See that when objects are moving (either on their own or if the player is moving them), they appear "laggy".

Expected behavior
Objects should appear to move smoothly, no matter what the physics framerate or the monitor refresh rate is.

Screenshots
None.

Environment
Linux Mint 20

Version
master

Additional context
There is a proposal to add this functionality to Godot itself: godotengine/godot-proposals#671

Since it is not currently in Godot, there is both an addon and a module to implement it.

[drwhut]

Fixed by adding frame interpolation directly to the engine using code from the comments of the proposal mentioned above.