fix(addon-webgl): delete VAOs on renderer dispose

Register vertex array objects for disposal in GlyphRenderer and
RectangleRenderer, matching existing buffer and program cleanup.

Fixes #5918

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

addons/addon-webgl/src/RectangleRenderer.ts

@@ -103,6 +103,7 @@ export class RectangleRenderer extends Disposable {
 
     // Create and set the vertex array object
     this._vertexArrayObject = gl.createVertexArray();
+    this._register(toDisposable(() => gl.deleteVertexArray(this._vertexArrayObject)));
     gl.bindVertexArray(this._vertexArrayObject);
 
     // Setup a_unitquad, this defines the 4 vertices of a rectangle