### 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)

glBindBuffer(GL_UNIFORM_BUFFER, 0);
	}
	glActiveTexture(GL_TEXTURE0 + GLES3::Config::get_singleton()->max_texture_image_units - 5);
	glBindTexture(GL_TEXTURE_2D, texture_storage->render_target_get_sdf_texture(p_to_render_target));
	{
		state.default_filter = p_default_filter;
		state.default_repeat = p_default_repeat;
	}
	Size2 render_target_size = texture_storage->render_target_get_size(p_to_render_target);
	glViewport(0, 0, render_target_size.x, render_target_size.y);
	r_sdf_used = false;
	int item_count = 0;
	bool backbuffer_cleared = false;
	bool time_used = false;
	bool material_screen_texture_cached = false;
	bool material_screen_texture_mipmaps_cached = false;
	Rect2 back_buffer_rect;
	bool backbuffer_copy = false;
	bool backbuffer_gen_mipmaps = false;
	bool update_skeletons = false;
	Item *ci = p_item_list;
	Item *canvas_group_owner = nullptr;
	state.last_item_index = 0;
	while (ci) {
		if (ci->copy_back_buffer && canvas_group_owner == nullptr) {
			backbuffer_copy = true;
			if (ci->copy_back_buffer->full) {
				back_buffer_rect = Rect2();
			} else {
				back_buffer_rect = ci->copy_back_buffer->rect;