resolve merge conflict with refactor of engine.readTexturePixels to engine extensions.

src/Engines/Extensions/engine.readTexture.ts

@@ -11,6 +11,9 @@ declare module "../../Engines/thinEngine" {
 
 ThinEngine.prototype._readTexturePixels = function(texture: InternalTexture, width: number, height: number, faceIndex = -1, level = 0, buffer: Nullable<ArrayBufferView> = null): ArrayBufferView {
     let gl = this._gl;
+    if (!gl) {
+        throw new Error ("Engine does not have gl rendering context.");
+    }
     if (!this._dummyFramebuffer) {
         let dummy = gl.createFramebuffer();