[2D physics] Random flipping of sprite when rotated.

[Sslaxx]

Godot version:
3.1 beta3.

OS/device including version:
Linux 64-bit. Unlikely to be different under another OS.

Issue description:
The player sprite uses a Raycast2D to detect the angle of the floor underneath it (PlayerPivot). It also has this code:

(to disable when in the air):

	if ($PlayerPivot.enabled):	# If in the air and the pivot/floor edge detectors are enabled, disable them.
		$PlayerPivot.enabled = false
		$FloorEdgeLeft.enabled = false
		$FloorEdgeRight.enabled = false
		rotation = 0			# Avoid the player character being at odd angles when in the air and returning to the ground.

(to enable when on the floor, and check/change rotation angle):

	if (!$"PlayerPivot".enabled):	# If on the ground and the pivot/floor edge detectors are not enabled, enable them.
		$"PlayerPivot".enabled = true
		$"FloorEdgeLeft".enabled = true
		$"FloorEdgeRight".enabled = true
	else:							# Make sure the player is angled to the ground.
		ground_normal = $"PlayerPivot".get_collision_normal ()
		ground_angle = (floor_normal.angle_to (ground_normal))
		rotation = (rotation if player_speed < 0.05 else ground_angle)

This usually works as expected - the player sprite "follows" the angle of the floor underneath it. But sometimes, for reasons not entirely clear, sometimes the sprite will flip (usually by 90 degrees, but I've noticed other angles too). It seems to do this especially when having stopped mid-way on a slope and started to run up or down it, or running off the end of a slope and landing on a higher platform. This appears to be completely at random and is not reliably reproducible as far as I have determined.

I am looking at using the collision info for the player sprite's collision shape directly instead of a Raycast, but I'm not sure if I'd still run into the same problem. I'm also not 100% sure this isn't my error.

Minimal reproduction project:
Sslaxx/Sonic_Outbreak@1ab26ed (https://github.com/Sslaxx/Sonic_Outbreak/tree/godot_sonic_engine)

[ghost]

If you're not having any issues with the vector signs juggling for a frame, then it could be related to this: #21020

I had issues with sprites when dealing with rotation and trying to use (-1, 1) scaling for mirroring. It doesn't seem to return what you would expect with certain rotations when you have inverted scaling.

[Sslaxx]

I hadn't noticed, but I'll see if I can do any more testing to find out.

[Sslaxx]

So far, using the version of Sonic Outbreak (the project I reported this issue as happening in) from #24050 (comment) with 3.2.2 I haven't noticed this issue.

I have noticed that jumping when moving no longer triggers the appropriate animation (when it did before) and incorrect jumping distances/speeds. Unsure at this stage if it's a bug in my code exposed by 3.2.2 (more likely), or a bug in 3.2.2.