Fix Premul Alpha, alpha

src/babylon.engine.ts

@@ -2005,7 +2005,7 @@
                     this._alphaState.alphaBlend = false;
                     break;
                 case Engine.ALPHA_PREMULTIPLIED:
-                    this._alphaState.setAlphaBlendFunctionParameters(this._gl.ONE, this._gl.ONE_MINUS_SRC_ALPHA, this._gl.ONE, this._gl.ONE_MINUS_SRC_ALPHA);
+                    this._alphaState.setAlphaBlendFunctionParameters(this._gl.ONE, this._gl.ONE_MINUS_SRC_ALPHA, this._gl.ONE, this._gl.ONE);
                     this._alphaState.alphaBlend = true;
                     break;
                 case Engine.ALPHA_COMBINE: