Do not use depth dimension when copying normal 2D textures of a 2d texture
array. It copies past the end of the buffer obtained via
ID3D11DeviceContext::Map().

Fixes a case which can be reproed with the payload disabled:

gn args out\debug --args="is_debug=true"
ninja -C out\debug angle_end2end_tests
out\debug\angle_end2end_tests ^
 --gtest_filter=Texture2DArrayCopy.SnormFormats* --gtest_catch_exceptions=0 ^
 --gtest_repeat=-1

Bug: angleproject:2865
Change-Id: Id9bc5489fa41749d1b1d2e87992166b1a0af76fc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1641247Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>