Improve ImageryLayer.cancelReprojections() handling

CHANGES.md

@@ -12,6 +12,7 @@
 - Fixed an issue where Plane doesn't rotate correctly around the main local axis. [#8268](https://github.com/CesiumGS/cesium/issues/8268)
 - Fixed clipping planes with non-uniform scale. [#9135](https://github.com/CesiumGS/cesium/pull/9135)
 - Fixed an issue where ground primitives would get clipped at certain camera angles. [#9114](https://github.com/CesiumGS/cesium/issues/9114)
+- Fixed a bug that could cause half of the globe to disappear when setting the `terrainProvider. [#9161](https://github.com/CesiumGS/cesium/pull/9161)
 - Fixed a crash when loading Cesium OSM buildings with shadows enabled. [#9172](https://github.com/CesiumGS/cesium/pull/9172)
 
 ### 1.73 - 2020-09-01

Source/Renderer/ComputeCommand.js

@@ -69,6 +69,14 @@ function ComputeCommand(options) {
    */
   this.postExecute = options.postExecute;
 
+  /**
+   * Function that is called when the command is canceled
+   *
+   * @type {Function}
+   * @default undefined
+   */
+  this.canceled = options.canceled;
+
   /**
    * Whether the renderer resources will persist beyond this call. If not, they
    * will be destroyed after completion.

Source/Scene/ImageryLayer.js

@@ -1235,6 +1235,10 @@ ImageryLayer.prototype._reprojectTexture = function (
         imagery.state = ImageryState.READY;
         imagery.releaseReference();
       },
+      canceled: function () {
+        imagery.state = ImageryState.TEXTURE_LOADED;
+        imagery.releaseReference();
+      },
     });
     this._reprojectComputeCommands.push(computeCommand);
   } else {
@@ -1268,6 +1272,11 @@ ImageryLayer.prototype.queueReprojectionCommands = function (frameState) {
  * @private
  */
 ImageryLayer.prototype.cancelReprojections = function () {
+  this._reprojectComputeCommands.forEach(function (command) {
+    if (defined(command.canceled)) {
+      command.canceled();
+    }
+  });
   this._reprojectComputeCommands.length = 0;
 };