Expose `ignoreDirectionalLights` attribute to be used in level scripts

[assertivist]

We have a flag used for the sights/UI, ignoreDirectionalLights, that makes it so that only ambient lighting is applied to a BSP object.

As a Level Designer, I would like to specify ignoreDirectionalLights = true or directionalLighting = false, in order to utilize this functionality for an accent or effect in a level.

[rherriman]

Took a different approach via #513.

It adds a glow property for materials, which is specified as a floating point number between 0 and 256 in ALF. The idea is that in a future with HDR/tone mapping/bloom/etc., local "intensity" for a color might actually exceed 1.0 via this glowing effect. Because we don't currently live in that future, any value over 0 will apply the same glow, essentially applying the base material color as-is without additional lighting calculations.

Note that we are currently sending the glow value along with the other material data in the vertex buffer(s), and for now, I had to do so as a float. My initial attempt used a uint8_t, but because of the way C++ pads structs, it caused problems when it came time to set vertex attribute pointers. You can actually still see artifacts of my original approach in the source code, but I'm leaving them as-is: if we add additional material properties in the future, it would be best to try to cram multiple properties (including glow) into a 32-bit int, much like how we combine specular and shininess.