layers: Fix problems with screenshot to get it working again
layers: Fix problems with screenshot to get it working again

- Set the layerCount member of VkImageSubresourceLayers
- Add image transition memory barriers (needed for some drivers)
- Add dispatch table to command buffer object created by this layer.
  This lets it get dispatched by a downstream layer, if present.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools