### 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](https://github.com/godotengine/godot/files/10678236/Particle2DColor.zip)

0, 0,
			0, 1,
			1, 1,
			1, 0
		};
		glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 8, qv, GL_STATIC_DRAW);
		glBindBuffer(GL_ARRAY_BUFFER, 0);
		glGenVertexArrays(1, &data.canvas_quad_array);
		glBindVertexArray(data.canvas_quad_array);
		glBindBuffer(GL_ARRAY_BUFFER, data.canvas_quad_vertices);
		glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, nullptr);
		glEnableVertexAttribArray(0);
		glBindVertexArray(0);
		glBindBuffer(GL_ARRAY_BUFFER, 0); 
	}
	{
		
		glGenBuffers(1, &data.particle_quad_vertices);
		glBindBuffer(GL_ARRAY_BUFFER, data.particle_quad_vertices);
		{
			
			const float qv[16] = {
				-0.5, -0.5,
				0.0, 0.0,
				-0.5, 0.5,
				0.0, 1.0,
				0.5, 0.5,
				1.0, 1.0,
				0.5, -0.5,
				1.0, 0.0