Re-evaluate problems with vertically offset glyphs on some fonts

[Tyriar]

There's a workaround in VS Code to tweak the default fonts used in the terminal as the font metrics always seemed to be a little wrong, shifting the font down such that underscores wouldn't show among other problems:

https://github.com/microsoft/vscode/blob/05519998aeb60a492478948e6e196586a24c04c6/src/vs/workbench/contrib/terminal/browser/terminalConfigurationService.ts#L114-L125

Another issue has come up with Source Code Pro, SauceCodePro Nerd Font in microsoft/vscode#204591 that shows a similar problem. We should look into how we calculate the bounding box in webgl and dive a little deeper into this problem:

• Would moving to TextMetrics solve these problems over the current approach?
• What textBaseline are we using and can we move to alphabetical?