[kihx] Put DepthFunc objects on RenderingContext.

- Remove DepthFunc interfaces from PixelProcessor.
- Handle DepthFunc objects in RenderingContext.
[kihx] Refactoring entire code.

- Split complex files into new files.
- Use FORCEINLINE macro.
- Revise unstyled code.
[kihx] Add ReentryAssert to verify thread-safety.

- Add threading.h.
- Add class ReentryAssert with macro REENTRY_ASSERT(limit).