ShaderChunk: Revert premultiplied_alpha_fragment deletion.

Author: mrdoob

src/renderers/shaders/ShaderChunk.js

@@ -81,6 +81,7 @@ import clearcoat_pars_fragment from './ShaderChunk/clearcoat_pars_fragment.glsl.
 import iridescence_pars_fragment from './ShaderChunk/iridescence_pars_fragment.glsl.js';
 import opaque_fragment from './ShaderChunk/opaque_fragment.glsl.js';
 import packing from './ShaderChunk/packing.glsl.js';
+import premultiplied_alpha_fragment from './ShaderChunk/premultiplied_alpha_fragment.glsl.js';
 import project_vertex from './ShaderChunk/project_vertex.glsl.js';
 import dithering_fragment from './ShaderChunk/dithering_fragment.glsl.js';
 import dithering_pars_fragment from './ShaderChunk/dithering_pars_fragment.glsl.js';
@@ -207,6 +208,7 @@ export const ShaderChunk = {
 	iridescence_pars_fragment: iridescence_pars_fragment,
 	opaque_fragment: opaque_fragment,
 	packing: packing,
+	premultiplied_alpha_fragment: premultiplied_alpha_fragment,
 	project_vertex: project_vertex,
 	dithering_fragment: dithering_fragment,
 	dithering_pars_fragment: dithering_pars_fragment,

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

@@ -0,0 +1,8 @@
+export default /* glsl */`
+#ifdef PREMULTIPLIED_ALPHA
+
+	// Get normal blending with premultipled, use with CustomBlending, OneFactor, OneMinusSrcAlphaFactor, AddEquation.
+	gl_FragColor.rgb *= gl_FragColor.a;
+
+#endif
+`;