PerspectiveCamera: getViewBounds(), getViewSize() (#777)

* PerspectiveCamera: getViewBounds(), getViewSize()

* Fix import

types/three/src/cameras/PerspectiveCamera.d.ts

@@ -1,4 +1,5 @@
 import { Camera } from './Camera.js';
+import { Vector2 } from '../math/Vector2.js';
 
 /**
  * Camera that uses {@link https://en.wikipedia.org/wiki/Perspective_(graphical) | perspective projection}.
@@ -148,6 +149,18 @@ export class PerspectiveCamera extends Camera {
      */
     getFilmHeight(): number;
 
+    /**
+     * Computes the 2D bounds of the camera's viewable rectangle at a given distance along the viewing direction.
+     * Sets minTarget and maxTarget to the coordinates of the lower-left and upper-right corners of the view rectangle.
+     */
+    getViewBounds(distance: number, minTarget: Vector2, maxTarget: Vector2): void;
+
+    /**
+     * Computes the width and height of the camera's viewable rectangle at a given distance along the viewing direction.
+     * Copies the result into the target Vector2, where x is width and y is height.
+     */
+    getViewSize(distance: number, minTarget: Vector2, maxTarget: Vector2): void;
+
     /**
      * Sets an offset in a larger frustum.
      * @remarks