Seperate mipmaps from images.

[HexDecimal]

The current way mipmaps are implemented is not good. They need to be their own separate object.

• [ ] Create a new "mipmapped image" struct.
• [ ] New C functions for mipmaps.
• [ ] Document C functions.
• [ ] Add C++ API.
• [ ] Document C++ API.
• [ ] Deprecate old image functions.

While I'm refactoring the image struct I should make it more consistent.

• [ ] Change images to use TCOD_ColorRGBA pixels.
• [ ] Public documentation for the image struct.
• [ ] C++ API for image structs.
• [ ] Document C++ API.