minor API refactor of for EvalLimit :
- Replaced EvalData and EvalVertexData classes with a simpler DataStream class that only
  accesses a single data stream, binds and unbinds it

- DataStream has both an input and an output version which avoids much of the const-ness
  const-related ambiguity of the previous design pattern

- Vertex, varying and face-varying data now all have a dedicate struct (VertexData, VaryingData, FaceVaryingData)
  as a way of gathering the various data-streams required to perform sampling

- renamd some "Buffers" into "Tables" for better naming consistency with Far