Negative scale particles are not shown

[QbieShay]

Godot version:
3.2 development -
6b5102c

OS/device including version:
OS: Linux 5.8.3-arch1-1
GPU: Radeon 590

Issue description:
Negative scale on 2D particle is useful for conveying the impression of particles rotating in 3D.
There are two issues for me:

1. can't set negative scale for 2D particles
2. when using a negative scale curve, particles disappear

Minimal reproduction project:
particle_scale.zip

[KoBeWi]

You can't set negative scale to particles, the value is clamped at 0. The curve probably tries to set it to negative, but it gets clamped too.

So it's not really a bug, but intended behavior (except the negative curve part). Maybe the restriction could be removed though.

[Calinou]

I can confirm this on commit 754db65 (3.x branch). This also occurs after switching to GLES2.

Negative scale appears to be allowed in master now, still.

[akien-mga]

Fixed by #53852.