CPUParticles: setting direction.y zeroes direction.x unless direction.z is non-zero

[cptchuckles]

Godot version

3.3.2-stable

System information

Artix Linux 5.12.12 :: i7-9750H 16GB GeForce 1650 Max-Q :: GLES 3

Issue description

Changing a CPUParticle's direction.y to anything other than zero will cause it to emit only along the Y-axis, even if direction.x is non-zero, except both direction.x and direction.z are non-zero.

Here is a video (link):

Steps to reproduce

Create a CPUParticle, give it a basic mesh
Set the Direction X to something
Then change Direction Y
Observe that the particle emits exclusively on the Y-axis.
Then change Direction Z
Observe that the Direction X value is once again respected

Minimal reproduction project

Here is the test scene from the video

main.tscn

[gd_scene load_steps=2 format=2]

[sub_resource type="CubeMesh" id=1]
size = Vector3( 0.2, 0.2, 0.2 )

[node name="main" type="Spatial"]

[node name="CPUParticles" type="CPUParticles" parent="."]
mesh = SubResource( 1 )
direction = Vector3( 11.706, 0, 0 )
spread = 0.0
initial_velocity = 3.35

[akien-mga]

Confirmed in 8f3ec04.

(GPU) Particles don't seem to have this issue on the other hand.

[akien-mga]

Fixed by #51565.