Support many, many panes

[daonb]

What's missing? Please describe your flow and where this feature will help.

There's a way to support many more panes.
Instead of creating a webgl context for each pane we need to use a single context
that grows as the user adds more panes.

Additional context
This change starts with the xterm-addon-webgl.
Either we work together the xterm team to upgrade the addon or we change its
interface in a non-breaking way or we'll be forced to copy the addon and work
on a seaprate fork.

[daonb]

I opened an issue over at xterm.js

[DmitryPorotov]

Fixed in https://github.com/tuzig/terminal7/tree/244b3a562f6519dc800cdb3cebd6cf604ac48930

[daonb]

Added virtual-webgl2 and it seems to be working