xtermjs: try to detect hardware support for webgl2

xtermjs: try to detect hardware support for webgl2

With the new WebGL renderer, chrome/chromium has buggy software
support for emulating this (see [0]), so we have to detect that
manually and prevent loading the add-on. This fixes the issue that on
chrome without HW-support, it would not always render every character.

Firefox does not have support for a software renderer and the
loading/detection throws an exception, falling back to the default
renderer.

0: https://github.com/xtermjs/xterm.js/issues/4574

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>