Adds some minor shader cleanups and fixes that blocks without available normal maps in the atlas were not correctly lit with normal mapping enabled.

* Sets the default background texture color for the normal texture atlas to r=0.5, g=0.5, b=1.0 which equals the unit vector [0.0, 0.0, 1.0] when being decoded in the chunk shader
* Reduces flickering light effect - no more ghosts turning your torches on and off to scare you at night