Determines the type of 
Material that is supported by a
MaterialAppearance instance.  This is a trade-off between
flexibility (a wide array of materials) and memory/performance
(required vertex format and GLSL shader complexity.
Members
    All materials, including those that work in tangent space, are supported.
This requires 
position, normal, st,
tangent, and bitangent vertex attributes.
    Only basic materials, which require just 
position and
normal vertex attributes, are supported.
    Materials with textures, which require 
position,
normal, and st vertex attributes,
are supported.  The vast majority of materials fall into this category.
