Passing alpha channel as background color will break rendering

[Tyriar]

VS Code issue: microsoft/vscode#204663

In VS Code setting "terminal.background": "#17171788" (or panel.background) will result in this:

Only happens with webgl

[AnouarTouati]

Setting allowTransparency to true in the TerminalOptions seems to fix the issue in the Xterm demo and in VS Code.

It seems like there is some color combinations that cause the cells to be completely opaque when allowTransparency is false, and these cells have padding, so they render on top of each other and so only half of a character is shown.
Some of the combinations :

• "#171717[00-99]"
• "#F75756[00-F1]"

There are some comments on how setting allowTransparency to true will cause issues with performance, stroked lines and other things.