Use correct image extent for image to buffer copies

Use correct image extent for image to buffer copies

The final stage of those tests (reading back to a host-mapped buffer)
would use an incorrect depth for the source image extent, when that is a
2D texture array. Because the test didn't factor in the image type, it
would set both the image extent and the subresource layerCount to the
number of layers being copied.

Using `getExtent3D` we get the appropriate behaviour.

Affects:
- dEQP-VK.api.copy_and_blit.*.3d_images.3d_to_2d*
- dEQP-VK.api.copy_and_blit.*.resolve_image.whole_array_image.*
- dEQP-VK.api.copy_and_blit.*.resolve_image.diff_image_size.
  dst_256_256_11*

Components: Vulkan

VK-GL-CTS issue: 729

Change-Id: I26109772c42286a741a38aa9c9cd8a42a5873adc