Update glCopyTexSubImage3D() to account for the requirements necessary
for VK_IMAGE_TYPE_3D.

Bug: angleproject:3765
Test: KHR-GLES2.texture_3d.copy_sub_image.rgba8
Test: angle_end2end_tests CopyTexImageTestES3
Change-Id: Ife3d768323d0cfe2a53e5ae4c47a0747d65981bc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1730637Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>