Unpack skip needs to be taken into account when determining which part
of the unpack buffer is read. This is now done in the out-of-bounds
check when validating texture upload calls.

Unpack skip code is removed from D3D9 backend, since skip is not
supported in GLES2.

BUG=angleproject:1411
TEST=angle_end2end_tests

Change-Id: I0db4db0877a352613c57e2820e5b650edb5a73ab
Reviewed-on: https://chromium-review.googlesource.com/352450Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>