Fix GL reference in headless WebGL test code. (tensorflow#1832)

* Fix GL reference in headless WebGL test code.

* Cleanup & README

tfjs-backend-nodegl/demo/README.md

@@ -0,0 +1,44 @@
+# MobileNet tfjs-backend-nodegl Demo
+
+*This is a very early demo to show how tfjs-backend-nodegl can be used for headless WebGL acceleration.*
+
+To run this demo, perform the following:
+
+1. Move into `tfjs-backend-nodegl` (parent directory of this demo folder):
+```sh
+$ cd tfjs-backend-nodegl
+```
+
+2. Build package and compile TypeScript:
+```sh
+$ yarn && yarn tsc
+```
+
+3. Move into the demo directory:
+```sh
+$ cd demo
+```
+
+4. Prep and build demo:
+```sh
+$ yarn
+```
+
+5. Run demo:
+```sh
+$ node run_mobilenet_inference.js dog.jpg
+```
+
+Expected output:
+```sh
+$ node run_mobilenet_inference.js dog.jpg
+Platform node has already been set. Overwriting the platform with [object Object].
+  - gl.VERSION: OpenGL ES 3.0 (ANGLE 2.1.0.9512a0ef062a)
+  - gl.RENDERER: ANGLE (Intel Inc., Intel(R) Iris(TM) Plus Graphics 640, OpenGL 4.1 core)
+  - Loading model...
+  - Mobilenet load: 6450.763924002647ms
+  - Coldstarting model...
+  - Mobilenet cold start: 297.92842200398445ms
+  - Running inference (100x) ...
+  - Mobilenet inference: (100x) : 35.75772546708584ms
+```

tfjs-backend-nodegl/demo/run_mobilenet_inference.js

@@ -22,13 +22,12 @@ const jpeg = require('jpeg-js');
 
 const backendNodeGL = require('./../dist/index');
 
-console.log(`  - gl.VERSION: ${
-    backendNodeGL.gl.getParameter(backendNodeGL.gl.VERSION)}`);
-console.log(`  - gl.RENDERER: ${
-    backendNodeGL.gl.getParameter(backendNodeGL.gl.RENDERER)}`);
+const gl = tf.backend().getGPGPUContext().gl;
+console.log(`  - gl.VERSION: ${gl.getParameter(gl.VERSION)}`);
+console.log(`  - gl.RENDERER: ${gl.getParameter(gl.RENDERER)}`);
 
 const NUMBER_OF_CHANNELS = 3;
-const PREPROCESS_DIVISOR = tf.scalar(255 / 2);
+const PREPROCESS_DIVISOR = 255 / 2;
 
 function readImageAsJpeg(path) {
   return jpeg.decode(fs.readFileSync(path), true);