move_and_slide is slower than with Godot Physics

[mihe]

It appears that version 0.7.0 (most likely #559) introduced a performance regression to move_and_slide, causing it to take roughly twice as long as before, leading to it now being slower when compared to Godot Physics.

Here's how to reproduce it:

1. Download this project: move_and_slide_regression.zip
2. Download debug symbols (if you're going to profile): godot-jolt_v0.8.0-stable_symbols.zip
3. Build Godot from source (if you're going to profile)
4. Run the main scene
5. Note the average delta displayed on the screen
6. Switch physics engine to Jolt
7. Run the main scene again
8. Note the average delta being higher

When picking out some average-looking physics ticks in Superluminal, the total time spent in move_and_slide in this particular scene is as follows:

Godot Physics (godotengine/godot@58f0cae): 4.74 ms
Godot Jolt (0.6.0-stable): 3.41 ms
Godot Jolt (0.7.0-stable): 5.70 ms
Godot Jolt (0.8.0-stable): 6.11 ms

(Note that these fluctuate quite a bit, so the difference between 0.7.0 and 0.8.0 seems to be within margin of error and probably not the result of further regression.)

When looking at move_and_slide more closely, it appears that Godot Jolt is returning collisions from body_test_motion that causes Godot to believe that some of them are not contacting the floor, causing it to run apply_floor_snap here, which ends up taking as much time as the slide iterations themselves.