Add all Real-ESRGAN models for background upsampler

[sebaro]

I get better results with RealESRGAN_x4plus than RealESRGAN_x2plus.
These commits add all models from Real-ESRGAN:

    parser.add_argument('--bg_upsampler', type=str, default='None',
            help='Background upsampler. Optional: RealESRGAN_x4plus | RealESRNet_x4plus | RealESRGAN_x4plus_anime_6B | RealESRGAN_x2plus | '
                'realesr-animevideov3 | realesr-general-x4v3')
    parser.add_argument('--face_upsample', action='store_true', help='Face upsampler after enhancement. Default: False')
    parser.add_argument('--bg_tile_size', type=int, default=0, help='Tile size for background sampler. Default: 0')
    parser.add_argument('--bg_tile_pad', type=int, default=10, help='Tile padding for background sampler. Default: 10')