layers: Update cmds allowed in 2nd CB subpass
layers: Update cmds allowed in 2nd CB subpass

If subpassContents are of type VK_SUBPASS_CONTENTS_SECONDARY_COMMAND_BUFFERS
then CmdNextSubpass and CmdEndRenderPass are accepatable cmds to be added to
the secondary cmd buffer.

Spec language is from section 7.4 Render Pass Commands: If contents is
VK_SUBPASS_CONTENTS_SECONDARY_COMMAND_BUFFERS, the contents are recorded in
secondary command buffers that will be called from the primary command
buffer, and vkCmdExecuteCommands is the only valid command on the command
buffer until vkCmdNextSubpass or vkCmdEndRenderPass.

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