Fix OsdUtilAdaptiveEvaluator concurrency issue

- add a limit evaluation method to EvalLimitController that allows
  client code to directly pass the output buffer without binding it
  to the Context (the call only computes vertex interpolation of a
  single sample)

- switch the OsdUtilAdaptiveEvaluator to use the new method from the controller
  and stop stomping member

- cleanup buffer and member variables no longer used

- cleanup initialization logic to be better aware of uniform / adaptive

- add some assert sanity checks in the cpuEvalLimitKernels

fixes #293