If the user repreatedly calls glTexImage with null data and the same size,
it is expected that the texture would re-fill itself with zero'd data.
TextureD3D and ImageD3D would no-op these calls when the texture is
already the right size.

TEST=conformance2/rendering/blitframebuffer-filter-outofbounds
BUG=angleproject:1635

Change-Id: I810b15922759a4acfc0ef7da198f068b81e9efc4
Reviewed-on: https://chromium-review.googlesource.com/524436
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>