Fixed Canvas Alpha RenderTexture issue. Fix phaserjs#5426

Author: photonstorm

src/gameobjects/rendertexture/RenderTexture.js

@@ -1286,7 +1286,7 @@ var RenderTexture = new Class({
 
             var matrix = this.camera.matrix;
 
-            ctx.globalAlpha = this.globalAlpha;
+            ctx.globalAlpha = alpha;
 
             ctx.setTransform(matrix[0], matrix[1], matrix[2], matrix[3], matrix[4], matrix[5]);