Fixed an issue with offset reference spaces

Turns out we weren't properly passing along the base transform to the new reference space, which would cause it to appear off in cases like `local-floor` or `bounded-floor` reference spaces on devices with real floor detection. (Devices that fell back to the emulated height for `local-floor` did OK.)

src/api/XRFrame.js

@@ -16,7 +16,6 @@
 import {PRIVATE as SESSION_PRIVATE} from './XRSession';
 import XRViewerPose from './XRViewerPose';
 import XRView from './XRView';
-import { mat4 } from 'gl-matrix';
 
 export const PRIVATE = Symbol('@@webxr-polyfill/XRFrame');
 

src/api/XRReferenceSpace.js

@@ -72,25 +72,11 @@ export default class XRReferenceSpace extends XRSpace {
 
     this[PRIVATE] = {
       type,
+      transform,
       originOffset : mat4.identity(new Float32Array(16)),
     };
   }
 
-  /**
-   * NON-STANDARD
-   * Called when this space's base pose needs to be updated
-   * @param {XRDevice} device
-   */
-  _onPoseUpdate(device) {
-    switch(this[PRIVATE].type) {
-      case 'viewer': 
-        this._baseMatrix = device.getBasePoseMatrix();
-        break;
-      default:
-        break;
-    }
-  }
-
   /**
    * NON-STANDARD
    * Takes a base pose model matrix and transforms it by the