### 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)

ci->canvas_group_owner = nullptr; 
		}
		if (!backbuffer_cleared && canvas_group_owner == nullptr && ci->canvas_group != nullptr && !backbuffer_copy) {
			texture_storage->render_target_clear_back_buffer(p_to_render_target, Rect2i(), Color(0, 0, 0, 0));
			backbuffer_cleared = true;
		}
		if (ci == canvas_group_owner) {
			if (update_skeletons) {
				mesh_storage->update_mesh_instances();
				update_skeletons = false;
			}
			_render_items(p_to_render_target, item_count, canvas_transform_inverse, p_light_list, r_sdf_used, true);
			item_count = 0;
			if (ci->canvas_group->blur_mipmaps) {
				texture_storage->render_target_gen_back_buffer_mipmaps(p_to_render_target, ci->global_rect_cache);
			}
			canvas_group_owner = nullptr;
			
			backbuffer_cleared = false;
		}
		if (backbuffer_copy) {
			if (update_skeletons) {
				mesh_storage->update_mesh_instances();
				update_skeletons = false;
			}
			
			_render_items(p_to_render_target, item_count, canvas_transform_inverse, p_light_list, r_sdf_used);
			item_count = 0;
			texture_storage->render_target_copy_to_back_buffer(p_to_render_target, back_buffer_rect, backbuffer_gen_mipmaps);
			backbuffer_copy = false;