If a storage already existed, it may have old data in it.  Check if the
image is dirty and copy the initialized memory into the storage before
copying data from the framebuffer.

TEST=conformance/textures/misc/copy-tex-image-and-sub-image-2d

BUG=angleproject:1815

Change-Id: Ic69c5519b2e09e4b62025b1bf413d2a71a4a2afb
Reviewed-on: https://chromium-review.googlesource.com/568410Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>