vk/pipeline: Add an extra flag for force-disabling the vertex shader

vk/pipeline: Add an extra flag for force-disabling the vertex shader

This way we can pass in a vertex shader and yet have the pipeline emit an
empty 3DSTATE_VS packet.  We need this for meta because we need to trick
the compiler into not deleting our inputs but at the same time disable the
VS so that we can use a rectlist.  This should go away once we actually get
SPIR-V.