layers:Refactor CmdDispatch()
layers:Refactor CmdDispatch()

Refactor CmdDispatch() to use pre/post pattern. Updated the DrawType
helper functions for use with Dispatch functions as well. For the
update helper, broke that out into a separate DrawDispatch helper
that does common work, and then a Draw helper that calls the
DrawDispatch helper and then does some extra work only relevant
for draw calls.

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