This path might have been broken some time back with the "setData"
path enabled.

This speeds up sRGB mipmap generation considerably.

Bug: chromium:1130678
Change-Id: Id1c22bc1cfb815339bad5955ce990fe9f48d8b5b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2432112Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>