Use the green channel from alpha maps instead of red.

More precision is used for the green channel in RGB565 and DXT texture formats, so this would improve the quality in those cases.

src/renderers/shaders/ShaderChunk.js

@@ -466,8 +466,7 @@ THREE.ShaderChunk = {
 
 		"#ifdef USE_ALPHAMAP",
 
-		"	vec4 texelAlpha = texture2D( alphaMap, vUv );",
-		"	gl_FragColor.a *= texelAlpha.r;",
+		"	gl_FragColor.a *= texture2D( alphaMap, vUv ).g;",
 
 		"#endif"