[kihx] Add ConsoleCommand and Thread.

- Add ConsoleCommand: It abstracts a global variable or function that can be control both the console and code paths.
- Add ConsoleCommandExecuter: It manages and executes ConsoleCommand.
- Add Thread: C++11 styled threading class (incomplete yet).
[kihx] Optimize scanline conversion.

- Reduce sorting AET cost exploiting reverse() instead of sort() if AET has only two elements.
[kihx] Add ConsoleVariable and ConsoleCommandExecuter::Help().

- Split ConsoleCommand into two classes: ConsoleCommand and ConsoleVariable.
- Add ConsoleCommandExecuter::Help().
[kihx] Add a grid scene and move depth buffering from the pixel processor to the output merger.

- Add a grid scene by rendering multiple meshes.
- Depth buffering moves to the output merger.
[rasterizer] Toggle frame update or not.
[kihx] Ready for multi-threaded rendering.

- Add a test scene to render multiple meshes.
- Disable early-z culling.
- Refine pipeline stages for thread-safety.
- Add foundation code of parallel drawing.