fix #5063: load WebGL addon in try block

fix #5063: load WebGL addon in try block

since under certain circumstances (e.g. using safari < 16) the addon
constructor already throws an exception, not only the loading later.

To allow falling back to the canvas renderer, move it into the try
block after WebGL detection.

This should fix the xterm.js console on Apple's iOS.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>