TextureD3D_3D::copySubImage worked around missing functionality in D3D11
for copying a framebuffer directly to a texture storage but didn't handle
the case of a texture storage already existing.  This caused the image to
have out-of-date data before the new data was copied into it.  Simply copy
the data from the storage back into the image before performing the copy
from the framebuffer and then copy back to the storage afterwards.

TEST=conformance2/textures/misc/copy-texture-image-webgl-specific.html

BUG=angleproject:1815

Change-Id: I308d6a1d3ecbc738f7d0e232bece433e6b353638
Reviewed-on: https://chromium-review.googlesource.com/567199
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>