Need to redesign FrameProcessor and FrameOperator

[TzuChieh]

Several notes:

• FrameOperator can be categorized into
  • local (pixel-by-pixel), e.g., simple tone mapping, scale
  • zone (neighbors may contribute to the final pixel), e.g., blur, bloom
  • global (the whole frame can affect the pixel), e.g., exposure like eye adaption
• FrameProcessor should be thread-safe (const methods) once constructed
• Post processing should be a separated operation with its own API
  • This way we no longer need to distinguish processed & raw frames in APIs
• Need better mechanism/spec to avoid processing non-updated region, which caused unwanted effects during asynchronous result queries in previous versions
  • Currently this is solved by hacking, see a36de40bfcc7608cb228f55d93d7bb532bc3c267