### 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)

backbuffer_gen_mipmaps = false;
			material_screen_texture_cached = true; 
			material_screen_texture_mipmaps_cached = backbuffer_gen_mipmaps;
		}
		if (backbuffer_gen_mipmaps) {
			texture_storage->render_target_gen_back_buffer_mipmaps(p_to_render_target, back_buffer_rect);
			backbuffer_gen_mipmaps = false;
			material_screen_texture_mipmaps_cached = true;
		}
		
		items[item_count++] = ci;
		if (!ci->next || item_count == MAX_RENDER_ITEMS - 1) {
			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;
		}
		ci = ci->next;
	}
	if (time_used) {
		RenderingServerDefault::redraw_request();
	}
	state.canvas_instance_data_buffers[state.current_data_buffer_index].fence = glFenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 0);
	
	reset_canvas();
	state.current_data_buffer_index = (state.current_data_buffer_index + 1) % state.canvas_instance_data_buffers.size();
	state.current_instance_buffer_index = 0;