layers: Fix for VkBufferImageCopy for depth/stencil

layers: Fix for VkBufferImageCopy for depth/stencil

According to the Vulkan spec:

"If the the calling command's VkImage parameter's format is not a
depth/stencil format, then VkBufferImageCopy::bufferOffset must be
a multiple of the format's element size"

Currently the image validation layer is checking unconditionally if
VkBufferImageCopy::bufferOffset is a multiple of the format's
element size and reporting error otherwise.

Added code that skips the alignment check if the VkImage parameter's
format is combined depth/stencil.

Change-Id: I92f2a67eee590fa3616df9da121aceec65204c24