Lights: change sun light size to be specified as angle

This is the angular diameter as seen from earth, which is between 0.526° and
0.545° in reality. Sharing the size with other light types did not make much
sense and meant the unit was unclear.

Differential Revision: https://developer.blender.org/D4819

intern/cycles/render/light.h

@@ -47,6 +47,7 @@ class Light : public Node {
 
   float3 dir;
   float size;
+  float angle;
 
   float3 axisu;
   float sizeu;