Fix CopyTexImage behavior when cubemap workaround is involved.

The bug is, we need to reset PIXEL_UNPACK_BUFFER as well as UNPACK params
in order to upload data to textures correctly.

BUG=804118
TEST=tests in the bug
R=piman@chromium.org
NOTRY=true

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I743f03cdc98b2c6993449ca615b246eb42ef4dfa
Reviewed-on: https://chromium-review.googlesource.com/892282
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532749}