library: implement refraction

This necessitated to rework how light properties for a material were
given. A material can have either reflectivity or transparency. This
changes the parsing of materials, using a `LightProperty` structure at
its core.

This is does not implement the true Fresnel equations to take into
account the amount of reflection that an incident goes through when
encountering a transparent object.

src/serialize/coefficient.rs

@@ -0,0 +1,3 @@
+pub fn default_identity() -> f32 {
+    1.
+}