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.