Expose Hybrid Image Manager

[taj-p]

See second commit. Stacks on top of #43.

• Exposes ImageManager from Vello Hybrid for public consumption
• Makes ImageManager required

This is required to enable custom use of Vello Hybrid, for example, for benchmarking per the below:

            // Prepare GPU resources
            let mut encoder = gpu
                .device
                .create_command_encoder(&wgpu::CommandEncoderDescriptor::default());
            let texture_view = gpu
                .texture
                .create_view(&wgpu::TextureViewDescriptor::default());

            // Build the scene
            {
                let image_manager = ImageManager::new(
                    &mut renderer,
                    &gpu.device,
                    &gpu.queue,
                    &mut encoder,
                    &mut cached_images,
                );
                let mut painter = VelloHybridScenePainter::new(&mut hybrid_scene, image_manager);
                painter.append_scene(item.scene.clone(), Affine::IDENTITY);
            }

            renderer
                .render(
                    &hybrid_scene,
                    &gpu.device,
                    &gpu.queue,
                    &mut encoder,
                    &render_size,
                    &texture_view,
                )
                .expect("Hybrid render failed");