This article was withdrawn on October 15, 2020, at the request of the journal editors, with agreement from the authors, owing to a substantial amount of unattributed or improperly cited text overlap with other sources. In accordance with Annual Reviews' commitment to transparency, the original PDF of the article remains available for download at https://www.annualreviews.org/doi/pdf/10.1146/annurev-vision-091517-034018.