Reorganize EvalStencilContext and EvalStencilController

Moved transient states (current vertex buffer etc) to controller.
ComputeContext becomes constant so that it's well suited for coarse-grain
parallelism on cpu.
EvalStencil Context & Controller code cleanup

    - fix some variable names (private vs. public)
    - implement constructors to guarantee initialized pointers (d'oh)
    - add a 'Reset' method to unbind buffers

Note: while the new contexts have been cleaned up, we now have a fair amount of duplicated code in the controllers...
 Reorganize EvalLimitContext and EvalLimitController

Moved transient states (current vertex buffer etc) to controller.
ComputeContext becomes constant so that it's well suited for coarse-grain
parallelism on cpu.

Client-facing API has changed slightly - limitEval example has been adjusted
Merge pull request #299 from Nazg-Gul/dev

Implement C-API accessor to evaluator topology