Destroy the PIXI objects to free GPU memory.

I read that it is necessary to call destroy() to clean GPU memory as it is
not done automatically by PIXI. It seems to run more smoothly with this
change on my machine.

Signed-off-by: Arnaud Fiorini <fiorini.arnaud@gmail.com>

timeline-chart/src/layer/time-graph-layer.ts

@@ -47,14 +47,14 @@ export abstract class TimeGraphLayer {
     }
 
     protected removeChildren() {
-        this.children.forEach(child => this.layer.removeChild(child.displayObject));
+        this.children.forEach(child => child.destroy());
         this.children = [];
     }
 
     protected removeChild(child: TimeGraphComponent) {
+        child.destroy();
         const idx = this.children.findIndex(c => c === child);
         idx && this.children.splice(idx, 1);
-        this.layer.removeChild(child.displayObject);
     }
 
     protected getPixels(ticks: number) {