feat(WebXR): Add support for Looking Glass display

Adds support for rendering to Looking Glass holographic display
via Looking Glass WebXR polyfill.

View transforms provided by the Looking Glass WebXR polyfill are
retrieved each frame and used to render into a quilt to be copied onto
the Looking Glass external display.

xref https://lookingglassfactory.com/webxr

xref https://docs.lookingglassfactory.com/keyconcepts/quilts

Sources/Rendering/OpenGL/RenderWindow/Constants.d.ts

@@ -1,6 +1,7 @@
 export declare enum XrSessionTypes {
 	HmdVR = 0,
 	MobileAR = 1,
+  LookingGlassVR = 2
 }
 
 declare const _default: {

Sources/Rendering/OpenGL/RenderWindow/Constants.js

@@ -1,6 +1,7 @@
 export const XrSessionTypes = {
   HmdVR: 0, // Head-mounted display (HMD), two-camera virtual reality session
   MobileAR: 1, // Mobile device, single-camera augmented reality session
+  LookingGlassVR: 2, // Looking Glass hologram display, N-camera virtual reality session
 };
 
 export default {

Sources/Rendering/OpenGL/RenderWindow/index.d.ts

@@ -232,19 +232,34 @@ export interface vtkOpenGLRenderWindow extends vtkOpenGLRenderWindowBase {
 	get3DContext(options: I3DContextOptions): Nullable<WebGLRenderingContext>;
 
 	/**
-	 *
+	 * Request an XR session on the user device with WebXR,
+   * typically in response to a user request such as a button press.
 	 */
-	startVR(): void;
+	startXR(): void;
+
+  /**
+   * When an XR session is available, set up the XRWebGLLayer
+   * and request the first animation frame for the device
+   */
+   enterXR(): void,
+
+   /**
+    * Adjust world-to-physical parameters for different viewing modalities
+    *
+    * @param {Number} inputRescaleFactor
+    * @param {Number} inputTranslateZ
+    */
+   resetXRScene(inputRescaleFactor: number, inputTranslateZ: number): void,
 
 	/**
-	 *
+	 * Request to stop the current XR session
 	 */
-	stopVR(): void;
+	stopXR(): void;
 
 	/**
 	 *
 	 */
-	vrRender(): void;
+	xrRender(): void;
 
 	/**
 	 *

Sources/Rendering/OpenGL/RenderWindow/index.js

@@ -477,6 +477,13 @@ function vtkOpenGLRenderWindow(publicAPI, model) {
             // No handling for non-eye viewport
             return;
           }
+        } else if (model.xrSessionType === XrSessionTypes.LookingGlassVR) {
+          const startX = viewport.x / glLayer.framebufferWidth;
+          const startY = viewport.y / glLayer.framebufferHeight;
+          const endX = (viewport.x + viewport.width) / glLayer.framebufferWidth;
+          const endY =
+            (viewport.y + viewport.height) / glLayer.framebufferHeight;
+          ren.setViewport(startX, startY, endX, endY);
         } else {
           ren.setViewport(0, 0, 1, 1);
         }
@@ -490,6 +497,11 @@ function vtkOpenGLRenderWindow(publicAPI, model) {
 
         publicAPI.traverseAllPasses();
       });
+
+      // Reset scissorbox before any subsequent rendering to external displays
+      // on frame end, such as rendering to a Looking Glass display.
+      gl.scissor(0, 0, glLayer.framebufferWidth, glLayer.framebufferHeight);
+      gl.disable(gl.SCISSOR_TEST);
     }
   };
 
@@ -1294,7 +1306,6 @@ const DEFAULT_VALUES = {
   defaultToWebgl2: true, // attempt webgl2 on by default
   activeFramebuffer: null,
   xrSession: null,
-  xrSessionType: null,
   xrReferenceSpace: null,
   xrSupported: true,
   imageFormat: 'image/png',