turnip: copy all layers specified in vkCmdCopyImage

When copying layered images we ignored .layerCount parameter.

Found in D3D11 game "Company Of Heroes 2"

It seems like it worked ok for some reason when there is only one mipmap, which is the case for dEQP-VK.api.copy_and_blit.core.image_to_image.array.array_to_array_whole. But with several mips it doesn't work.

Marked as WIP because this change doesn't fix the issue in the game. It works for a standalone case modeled after what game does, but in a trace of the game I see subsequent `vkCmdCopyImage`s of higher mips corrupt previous mips (turn them black). (there are mip_levels number of `vkCmdCopyImage` each copying whole mip level)