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.

examples/colorful.yaml

@@ -47,6 +47,9 @@ objects:
         r: 1.0
         g: 1.0
         b: 1.0
+      # Optional fields (go together)
+      #transparency: 0.5
+      #index: 1.5
     texture:
       type: uniform
       color:
@@ -71,7 +74,7 @@ objects:
         g: 1.0
         b: 1.0
       # Optional field
-      # reflectivity: 0.0
+      #reflectivity: 0.0
     texture:
       type: uniform
       color:

examples/scene.yaml

@@ -35,7 +35,6 @@ objects:
       type: uniform
       diffuse: {r: 0.5, g: 0.5, b: 0.5}
       specular: {r: 1., g: 1., b: 1.}
-      reflectivity: 0.5
     texture:
       type: uniform
       color: {r: 0.25, g: 0.5, b: 1.}