Add comments to WebGLMorphtargets.js as well

src/renderers/webgl/WebGLMorphtargets.js

@@ -98,6 +98,9 @@ function WebGLMorphtargets( gl ) {
 
 		}
 
+		// GLSL shader uses formula baseinfluence * base + sum(target * influence)
+		// This allows us to switch between absolute morphs and relative morphs without changing shader code
+		// When baseinfluence = 1 - sum(influence), the above is equivalent to sum((target - base) * influence)
 		var morphBaseInfluence = geometry.morphTargetsRelative ? 1 : 1 - morphInfluencesSum;
 
 		program.getUniforms().setValue( gl, 'morphTargetBaseInfluence', morphBaseInfluence );