Display real FPS in the 3D editor instead of estimating based on CPU/GPU time

[Calinou]

This reverts to the 3.x method for displaying FPS in the View Frame Time panel. This has two consequences:

• The reported FPS will no longer go past V-Sync or framerate limiters.
• Unfocusing the window will display a low framerate (10 FPS by default), since the FPS is actually being limited by the engine's low-processor mode. In general, in this situation, the current FPS estimate can't be expected to be accurate as the GPU will often downclock as a result of the low GPU utilization.

Constant redrawing is still forced while the panel is visible, so there's no risk of having low-processor mode interfere.

The engine's FPS readout is updated less smoothly than the current method, but this will be improved if https://github.com/godotengine/godot/pull/63356 is merged.

On the images below, the actual FPS (and GPU power consumption) is identical, but since I use a 120 Hz display with V-Sync enabled, the engine cannot go past 120 FPS:

Before	After

• See https://github.com/godotengine/godot/issues/69218.