Integrate experimental library into core library

[mogemimi]

I have the following plan:

• Reimplement some experimental library, such as sprite renderer, shape/primitive renderer, easings, particle system, post effects and font renderer.
• Integrate experimental library (e.g. libpomdog_experimental.a) into core library (e.g. libpomdog.a). Btw, some libraries have already been moved into include/Pomdog/Experimental.
• Remove the obsolete Pomdog.Experimental library.

The current repository structure is the following,

- experimental/Pomdog.Experimental/ (= included in `libpomdog_experimental.a`)
  - Particle2D/
  - Skeletal2D/
  - ...
- include/Pomdog/Experimental/ (= included in `libpomdog.a`)
  - Graphics/
  - Image/
  - TexturePacker/

the old experimental library will be removed in the future as follows.

- include/Pomdog/Experimental/
  - Graphics/
  - Image/
  - Particle2D/
  - Skeletal2D/
  - TexturePacker/
  - ...

Progress / TODOs

• [x] Support Metal fully using preset shaders
  • https://github.com/mogemimi/pomdog/commit/f63ba4b5c8cf4af5990b8e03564eac111e69885a
  • https://github.com/mogemimi/pomdog/commit/e1f5fbfeb3d300de2232e6bb60cdbd4712099da9
  • https://github.com/mogemimi/pomdog/commit/3c4f69f233a585732467b56aebb387a6aeb48a43
  • https://github.com/mogemimi/pomdog/commit/7a4cb1c41dbbd37164c11393ba124f3535cdf918
  • https://github.com/mogemimi/pomdog/commit/29a666902d2dfa13effb74b6e427038ad9c5f4f1
  • https://github.com/mogemimi/pomdog/commit/afae4d3c29f56a9b61ea7d80dc7b8b2aaa181dd5
  • https://github.com/mogemimi/pomdog/commit/466f8c915fc3cb62520629e187b941385e1c3aaf
• [ ] Reimplementation
  • [x] Sprite batch renderer (https://github.com/mogemimi/pomdog/pull/30)
    • [x] Emulate glDrawArraysInstancedBaseInstance and glDrawElementsInstancedBaseInstance on OpenGL <= 4.1 for Mac (https://github.com/mogemimi/pomdog/commit/57a1c94591cd75f839cd8e14f2ae60984318ec1f)
    • [x] Optimization
    • [x] Resign API (https://github.com/mogemimi/pomdog/pull/30)
    • [x] Unify SpriteBatchRenderer and SpriteBatch (https://github.com/mogemimi/pomdog/pull/30)
  • [ ] TrueType font renderer
    • [ ] Optimization: Change surface format from RGBA8888 to R8
  • [ ] Primitive/shape renderer
  • [ ] Easings
  • [ ] Actions
  • [ ] Particle System 2D
  • [ ] Simple 2D Game Engine
• [ ] Integrating experimental library into core library.
  • [x] Moving Sprite, Primitive, Font features (https://github.com/mogemimi/pomdog/commit/5c4fbd3cd6d26e8e4456fd096622145c600087ef)
  • [ ] Particle System 2D
  • [x] MagicaVoxel exporter/importer (https://github.com/mogemimi/pomdog/commit/fcef973cead3869cb04794c9b0db38e068be2bc5)
  • [x] Easings (https://github.com/mogemimi/pomdog/commit/f68e14f936f6f18709d6e2de16b37ec4b2b51629)
  • [ ] Skeletal Animation, Skinned Mesh 2D
  • [ ] Entity-component system
• [ ] Removing the obsolete Pomdog.Experimental library.