Editor: Don't overwrite editor camera in Viewport.

[Mugen87]

This PR fixes a subtle bug in Viewport. When the viewport camera is changed, the viewportCameraChanged handler assigns the new camera to camera and thus overwrites the references to the editor's default perspective camera.

This actually breaks the resizing logic which is now encapsulated in updateAspectRatio(). To avoid this and for clarity reasons, it's best when the camera variable in Viewport always points to editor.camera.

[mrdoob]

Thanks!