libgui: Added stub for ModelDrawable

The first goal of this branch is to try out the Open Asset Import
Library and see if it can be used reliably for importing 3D models of
various formats and drawing them with animations.
Fixed|libdeng2: Resolved ambiguous method

size() comes from both base classes.
libgui|GLBuffer: Added a 3D vertex with tangent-space
libgui|ModelDrawable: Basic model parsing and preparation

Todo: Bones, animation.
Tests|GL: Added a MD2 test model for GLSandbox

Getting started with ModelDrawable.
qmake: Improved dep file for Open Asset Import Library

This one allows using a custom-built libassimp.
libgui|ModelDrawable: Working on data imported by libassimp
libdeng2: Added min/max functions with three arguments
libgui|GLBuffer: Drawing is a const operation
libgui|ModelDrawable: Simple drawing and bounding box

Here we're assuming the mesh needs no further transforming. A single
GLProgram is used to draw all meshes.
GLSandbox: Draw the loaded model
libgui|Debug: Print source file/line when a GL error occurs
libgui|GLUniform: Added support for matrix arrays
libgui: Added vertex attributes for bone IDs and weights
libgui|ModelDrawable: Rough implementation of skeletal animation

Skeletal animation is carried out using an array of bone matrices. Each
vertex of the mesh is given a set of bone IDs and weights, which are
then applied in the vertex shader.

Todo: Cleanup and removal of redundant O(n) lookups from Assimp data structures.
Tests|GLSandbox: Loading and rendering an MD5 model (without textures)

At the moment this tests the basic functionality of ModelDrawable:
loading a model with animations and rendering the geometry with skinning
applied.
Refactor|libgui: Cleanup and join meshes into a single GL buffer
Tests|GLSandbox: Shader tweak
Tests|GLSandbox: Added textures for the MD5 test model
libgui|ModelDrawable: Atlas for model textures