Implement basic cycles light handling. (#164)

Cycles lights are implemented using the node tree, and this patch
doesn't attempt to handle all the possibilities that enables.

However, it does succeed in exporting the basic options you can tweak
in the light settings panel, including:

- RGB Color (no alpha)
- Energy
- Negative Light (for strength < 0)
- Shadows enabled/disabled

This also removes the use_sphere check as cycles lights do not support
limited distance or use_sphere.

Fixes #150.

(cherry picked from commit b57cee82f3)

tests/reference_exports/material_cycle/material_anistropy.escn

@@ -268,12 +268,11 @@ surfaces/0 = {
 
 [node name="Point" type="DirectionalLight" parent="."]
 
-light_specular = 1.0
 light_energy = 1.0
 light_color = Color(1.0, 1.0, 1.0, 1.0)
-shadow_color = Color(0.0, 0.0, 0.0, 1.0)
 transform = Transform(0.701062, -0.562597, -0.438174, 0.452107, -0.124517, 0.88323, -0.551463, -0.8173, 0.16706, -2.09735, 4.28268, -0.137035)
 shadow_enabled = true
+light_negative = false
 
 [node name="object_with_uv" type="MeshInstance" parent="."]
 

tests/reference_exports/material_cycle/material_cycle.escn

@@ -1699,27 +1699,25 @@ material/0 = SubResource(39)
 
 [node name="Spot001" type="SpotLight" parent="."]
 
-light_specular = 1.0
 light_energy = 1.0
 light_color = Color(1.0, 1.0, 1.0, 1.0)
-shadow_color = Color(0.0, 0.0, 0.0, 1.0)
 spot_angle = 17.4
 spot_angle_attenuation = 1.25
 spot_range = 25.0
 transform = Transform(0.629608, 0.0765798, 0.77313, -0.776913, 0.06206, 0.626542, 0.0, -0.99513, 0.0985693, 5.32508, 4.74617, 0.487329)
 shadow_enabled = true
+light_negative = false
 
 [node name="Spot" type="SpotLight" parent="."]
 
-light_specular = 1.0
 light_energy = 1.0
 light_color = Color(1.0, 1.0, 1.0, 1.0)
-shadow_color = Color(0.0, 0.0, 0.0, 1.0)
 spot_angle = 24.3
 spot_angle_attenuation = 1.25
 spot_range = 25.0
 transform = Transform(0.633641, 0.273452, -0.723687, 0.117184, 0.890727, 0.439173, 0.764701, -0.363082, 0.532357, -4.6068, 2.54209, 4.80618)
 shadow_enabled = true
+light_negative = false
 
 [node name="Plane" type="MeshInstance" parent="."]
 

tests/reference_exports/material_cycle/material_normal.escn

@@ -844,12 +844,11 @@ material/0 = SubResource(12)
 
 [node name="Lamp" type="DirectionalLight" parent="."]
 
-light_specular = 1.0
-light_energy = 1.0
+light_energy = 0.04
 light_color = Color(1.0, 1.0, 1.0, 1.0)
-shadow_color = Color(0.0, 0.0, 0.0, 1.0)
 transform = Transform(-0.404791, -0.439839, 0.801677, 0.221124, 0.803611, 0.552553, -0.88727, 0.400938, -0.228035, 4.07625, 1.70784, -1.00545)
 shadow_enabled = true
+light_negative = false
 
 [node name="test_displacement" type="MeshInstance" parent="."]