Added clamping to spotLight term in shaders.

This is necessary for OpenGL where pow(0,0) is undefined and can be negative (in DirectX you just get zero), leading to negative light creating black areas artefacts.

Fixes #1805 and #2615 (related already fixed #998).

build/three.js

@@ -14281,7 +14281,7 @@ THREE.ShaderChunk = {
 
 				"if ( spotEffect > spotLightAngle[ i ] ) {",
 
-					"spotEffect = pow( spotEffect, spotLightExponent[ i ] );",
+					"spotEffect = max( pow( spotEffect, spotLightExponent[ i ] ), 0.0 );",
 
 					"float lDistance = 1.0;",
 					"if ( spotLightDistance[ i ] > 0.0 )",
@@ -14640,7 +14640,7 @@ THREE.ShaderChunk = {
 
 				"if ( spotEffect > spotLightAngle[ i ] ) {",
 
-					"spotEffect = pow( spotEffect, spotLightExponent[ i ] );",
+					"spotEffect = max( pow( spotEffect, spotLightExponent[ i ] ), 0.0 );",
 
 					// diffuse
 
@@ -25447,7 +25447,7 @@ THREE.ShaderUtils = {
 
 							"if ( spotEffect > spotLightAngle[ i ] ) {",
 
-								"spotEffect = pow( spotEffect, spotLightExponent[ i ] );",
+								"spotEffect = max( pow( spotEffect, spotLightExponent[ i ] ), 0.0 );",
 
 								// diffuse
 

src/extras/ShaderUtils.js

@@ -365,7 +365,7 @@ THREE.ShaderUtils = {
 
 							"if ( spotEffect > spotLightAngle[ i ] ) {",
 
-								"spotEffect = pow( spotEffect, spotLightExponent[ i ] );",
+								"spotEffect = max( pow( spotEffect, spotLightExponent[ i ] ), 0.0 );",
 
 								// diffuse
 

src/renderers/WebGLShaders.js

@@ -610,7 +610,7 @@ THREE.ShaderChunk = {
 
 				"if ( spotEffect > spotLightAngle[ i ] ) {",
 
-					"spotEffect = pow( spotEffect, spotLightExponent[ i ] );",
+					"spotEffect = max( pow( spotEffect, spotLightExponent[ i ] ), 0.0 );",
 
 					"float lDistance = 1.0;",
 					"if ( spotLightDistance[ i ] > 0.0 )",
@@ -969,7 +969,7 @@ THREE.ShaderChunk = {
 
 				"if ( spotEffect > spotLightAngle[ i ] ) {",
 
-					"spotEffect = pow( spotEffect, spotLightExponent[ i ] );",
+					"spotEffect = max( pow( spotEffect, spotLightExponent[ i ] ), 0.0 );",
 
 					// diffuse