Simplify the pipeline_stage structure

Simplify the pipeline_stage structure
	- remove input/output fields, input tracking removed.
	- remove state fields, the validate function now called
	  on every statechange.
	- add an explicit 'create' function.

Add in code to build vertex program to implement current t&l state.  Still
disabled, but turn on with a #define in t_vp_build.h.