ShaderChunk: Document saturate ifdefs.

src/renderers/shaders/ShaderChunk/common.glsl.js

@@ -8,7 +8,7 @@ export default /* glsl */`
 #define EPSILON 1e-6
 
 #ifndef saturate
-// tonemapping_pars_fragment may have defined saturate() already
+// <tonemapping_pars_fragment> may have defined saturate() already
 #define saturate(a) clamp( a, 0.0, 1.0 )
 #endif
 #define whiteComplement(a) ( 1.0 - saturate( a ) )

src/renderers/shaders/ShaderChunk/tonemapping_pars_fragment.glsl.js

 export default /* glsl */`
 #ifndef saturate
-	#define saturate(a) clamp( a, 0.0, 1.0 )
+// <common> may have defined saturate() already
+#define saturate(a) clamp( a, 0.0, 1.0 )
 #endif
 
 uniform float toneMappingExposure;