Godot version

4.0 beta 17

System information

Windows 10

Issue description

Setting the color attribute in ParticleProcessMaterial for GPUParticles2D does not work with compatibility renderer
(works with forward+)

Steps to reproduce

Set renderer to compatibility
Create a GPUParticles2D
Add ParticleProcessMaterial and set Color attribute (creating a particle shader and setting the COLOR is the same)

Minimal reproduction project

Particle2DColor.zip