download and use xterm-addon-webgl as renderer

download and use xterm-addon-webgl as renderer

since xterm.js 5.0.0, the previous default canvas renderer is not
built-in anymore and only recommended as a fallback. The new recommended
renderer is this webgl one, so use it with a fallback to the default
built-in dom renderer (which is slower, but still functional).

to use it we have to load the addon after `terminal.open` so move the
`open` call to before the addon loading. While we're touching that line,
remove the superfluous second 'true' parameter that did not do anything.

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