Handle gaps in FS inputs.
Handle gaps in FS inputs.

Added logic to handle when user varyings are written by the VS but not read by the FS.
Also moved SBE command building from draw-time to pipeline-create.

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