/**
* Holds material information that can be used for lighting. Returned by all czm_getMaterial functions.
*
* @name czm_material
* @glslStruct
*
* @property {vec3} diffuse Incoming light that scatters evenly in all directions.
* @property {float} specular Intensity of incoming light reflecting in a single direction.
* @property {float} shininess The sharpness of the specular reflection. Higher values create a smaller, more focused specular highlight.
* @property {vec3} normal Surface's normal in eye coordinates. It is used for effects such as normal mapping. The default is the surface's unmodified normal.
* @property {vec3} emission Light emitted by the material equally in all directions. The default is vec3(0.0), which emits no light.
* @property {float} alpha Opacity of this material. 0.0 is completely transparent; 1.0 is completely opaque.
*/
struct czm_material
{
vec3 diffuse;
float specular;
float shininess;
vec3 normal;
vec3 emission;
float alpha;
};材质信息，一般使用 czm_getMaterial 函数来创建。
vec3
漫反射入射光。
float
入射光强度。
float
镜面反射强度。高值会产生更明显的镜面高光效果。
vec3
观察坐标系（eye-coordinate）中的法向量。
vec3
自发光色。默认值是 vec3(0.0)，即不发光。
float
材质的透明度。0.0 是完全透明，1.0 是完全不透明。
czm_getMaterial函数