rotating_cube: Add various data

Cube mesh data & eye position/target variables
rotating_cube: Set modelview matrix in drawScene
rotating_cube: Add init_cube method

Fills GL buffers with cube data
rotating_cube: Add bind_cube method
rotating_cube: Fix init/bind_cube
rotating_cube: Add rotation variable
gl-util: Add get_shader (from learningwebgl.com)
rotating_cube: Add plain shaders

Compilation/use of these shaders isn't implemented yet