Refactor the CPU copy texture code into a function in renderer utils. BUG=angleproject:1932 Change-Id: Iab79f2a09c2d8a85d2a9dde34acf4d2151072c2b Reviewed-on: https://chromium-review.googlesource.com/612561 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>