Memory growth due to Marker object calling update function

[Naxior]

Description
When the Marker object calls the update function, the THREE.Mesh instance(color meterial) memory is not released.

• Library Version: latest (develop branch)
• ROS Version: Noetic
• Platform / OS: Ubuntu 20.04

Steps To Reproduce

...
      case ROS3D.MARKER_TEXT_VIEW_FACING:
          this.traverse (function (child){
              if (child instanceof THREE.Mesh) {
                  child.material = colorMaterial;
              }
          });
          break;
      case ROS3D.MARKER_MESH_RESOURCE:
          var meshColorMaterial = null;
          if(message.color.r !== 0 || message.color.g !== 0 ||
             message.color.b !== 0 || message.color.a !== 0) {
              meshColorMaterial = this.colorMaterial;
          }
          this.traverse (function (child){
              if (child instanceof THREE.Mesh) {
                  child.material = meshColorMaterial;
              }
          });
          break;
...

Expected Behavior
Open Chrome developer tool and click Memory tag, memory stable

Actual Behavior
Memory is growing

Quick fix

...
      case ROS3D.MARKER_TEXT_VIEW_FACING:
          this.traverse (function (child){
              if (child instanceof THREE.Mesh) {
                  child.material.dispose();
                  child.material = colorMaterial;
              }
          });
          break;
      case ROS3D.MARKER_MESH_RESOURCE:
          var meshColorMaterial = null;
          if(message.color.r !== 0 || message.color.g !== 0 ||
             message.color.b !== 0 || message.color.a !== 0) {
              meshColorMaterial = this.colorMaterial;
          }
          this.traverse (function (child){
              if (child instanceof THREE.Mesh) {
                  if (child.material !== null) {
                      child.material.dispose();
                  }
                  child.material = meshColorMaterial;
              }
          });
          break;
...

[MatthijsBurgh]

@Naxior I am not able to test it right now. But could you create a PR in the meantime? You found the bug, so it is better, when you write the commit.