lessons/04: Improve README
lessons/04: Add more guidance
lessons/04: Provide more guidance in README
gc: Define stub func gc_mat4_imul()
lessons/04: Provide more guidance in README
lessons/04: Fix funcname type in README
lessons/05-spinning-cube/README.txt: Document requirements from lesson 4 solution
lessons/04: Remove stale documentation in README
gc: Rename gc_drawable_t::model_mat4 -> model_txform

This matches the name used in the lesson 04 README.
lessons/06: Add new lesson, incomplete and work-in-progress
Fix bug in matrix multiplication
Simplify math library usage: inline functions that operate on values

By making the matrix functions static inlines, the compiler can do
something sane when passing a mat4 by value, making the library much
easier to use.
gc_math: Add mat4_axis_angle function to construct a rotation matrix
gc_prog: Add wrapper for glGetUniformLocation
gc_drawable: Initialize the model matrix to an identity
gc_drawable: Add and use the u_model_txform uniform
game: Simple animation