libdeng2|BitField: Added assignment operator, method to get all element ids
libgui|GLState: Basic implementation of stacked GL state manipulation

GLStates can either be created on demand with GLState::push(), or
one can keep a GLState instance around for repeating use.

(Note that Direct3D uses an object-based state system compatible with
GLState.)
Refactor|libgui: Streamlined GLState's interface somewhat
libgui: Added Drawable, a combination of buffers, programs and states

Drawable is a tool for managing the interconnections between buffers,
programs and states. Possible use cases:

- draw a single buffer with a program using the current GL state

- draw a single buffer with one of many alternative programs

- draw multiple buffers with the same program

- draw some buffers with a custom GL state and the rest with the
  current state

- draw a mixture of multiple buffers and programs, each with a
  custom state (e.g., a set of 3D sub-models representing an object
  in the game)
Cleanup
libdeng2|libgui: Cleanup
libgui: Drawable is an AssetGroup

The asset group determines when it is allowed to draw the Drawable.
libgui: Cleanup
Refactor: Rectangle<> separate size type, Rectanglei uses unsigned size

Coherently using unsigned ints for sizes when dealing with integer
vectors and rectangles.

Sizes can never be negative.
libgui: Implemented GLTarget (FBOs); target is part of GLState

Totally untested.
libdeng2|ByteRefArray: Added a default constructor, data accessors
libgui: Cleanup, apidoc improvements
libgui: Added de::Image, a wrapper that extends QImage

The pixel formats supported by QImage are only partially compatible
with OpenGL (ES). de::Image defines a full set of GLES friendly
formats that can be used to pass raw pixel data (or QImages) to
GLTexture.
libgui|GLTexture: 2D textures, cube maps, automatic/manual mipmaps

Implemented GLTexture with management of 2D textures and cube maps.
Image content can be uploaded to any mipmap level. Mipmaps can be
generated automatically, by request, or set up entirely manually.
libgui: API doc improvements
libdeng2|Id: Added isNone() method and a qHash() function
libdeng2|Rectangle: Added expanded(), fromSize() constructor, union
libdeng2|Vector: Added Vector4ub typedef, suitable for RGBA colors
libgui|DisplayMode: Note about why it uses a C API
libgui|GLTexture: Notify specialized textures about texture binding

When a texture is taken into use, a specialized texture may need to
take some preparatory steps.