Improved engine option typing

Typed engine options so they inherit from WebGLContextAttributes and fixed issue where options.antialias was always ignored, even when the antialias parameter in the constructor call wasn't passed

src/babylon.engine.ts

@@ -183,6 +183,10 @@
         public textureLOD: boolean;
         public drawBuffersExtension;
     }
+    
+    export interface EngineOptions extends WebGLContextAttributes{
+        limitDeviceRatio?: number;
+    }
 
     /**
      * The engine class is responsible for interfacing with all lower-level APIs such as WebGL and Audio.
@@ -383,13 +387,16 @@
          * @param {boolean} [antialias] - enable antialias
          * @param options - further options to be sent to the getContext function
          */
-        constructor(canvas: HTMLCanvasElement, antialias?: boolean, options?: { antialias?: boolean, preserveDrawingBuffer?: boolean, limitDeviceRatio?: number }, adaptToDeviceRatio = true) {
+        constructor(canvas: HTMLCanvasElement, antialias?: boolean, options?: EngineOptions, adaptToDeviceRatio = true) {
             this._renderingCanvas = canvas;
 
             this._externalData = new StringDictionary<Object>();
 
             options = options || {};
-            options.antialias = antialias;
+
+            if(antialias != null){
+                options.antialias = antialias;
+            }
 
             if (options.preserveDrawingBuffer === undefined) {
                 options.preserveDrawingBuffer = false;
@@ -2846,4 +2853,4 @@
             }
         }
     }
-}
+}