Use shaderParams chunk type also for params set in shader nodes

This eliminates duplicate code.
Remove paramsStack check from shaderParams chunk

paramsStack is always set in the cores of the chunks that make it to the
draw lists. Default chunk is marked as empty so it is not added to any
lists.
Merge pull request #323 from NVIDIA/reuse-shaderparams-chunk

Use shaderParams chunk type also for params set in shader nodes