libdeng2|Asset: Improved public interface of de::Asset
libgui|GLBuffer: Vertex attribute indices
libgui|GLShader: Shader instances are reference counted

Shaders can be shared by multiple programs; reference counting solves
lifetime management in this situation.
libgui|GLTexture: Binding the texture to a sampler unit
libgui|GLUniform: Applying uniform values in a GL program
libgui|GLProgram: Basic implementation of GL programs

Binding vertex attributes, applying uniform values, attaching shaders
and linking the program.