Implement the CPU fallback for CopyTextureCHROMIUM on OpenGL. BUG=angleproject:1932 Change-Id: Iabc1a3e361d66313dc16bf19b392402b7836f8a5 Reviewed-on: https://chromium-review.googlesource.com/612562 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>