Feedback sprite packer can optimise assetbundle build time in 2019.4.40

profiling cpu time of assetbundle build on windows 10 shows at least on 2019.4.40, both sprite atlas and sprite packer contribute some time on every assetbundle. in our project, 5w+ assetbundles and 5GB+ ios/android assetbundle size, use sprite atlas building assetbundle from 0 will cost 6 hours, use sprite packer will cost 3.5 hours, set sprite packer mode to disabled will cost 2 hours