Add 'uv' attribute
Increase cube vertex count from 8 to 24 for texturing purposes

Three are at the same position, but for different faces so they can be
textured uniquely. Based on
https://developer.mozilla.org/en-US/docs/Web/WebGL/Creating_3D_objects_using_WebGL

"If you wonder why we need 24 vertices, and not just 8, it is because
each corner belongs to three faces of different colours, and a single
vertex needs to have a single specific color - therefore we will create
3 copies of each vertex in three different colors, one for each face."
Set whole-image UV coordinates from MDN cube example

https://developer.mozilla.org/en-US/docs/Web/WebGL/Using_textures_in_WebGL
Correct comments for our camera perspective: back/front and left/right swapped
Add correct front face UV coords
Refactor into setCubeFaceUV, cubeFaceNameIndex
Replace test skin with viking.png

From https://github.com/maxogden/minecraft-skin
More distinct features, makes for easier testing
Rotate UV so hat is up
Head now completely renders, set UVs for all cube faces