Cache all colors used by the terminal

[Tyriar]

IColor objects get created and thrown away, for example here:

https://github.com/microsoft/vscode/blob/22d334c8c1826b5b2f4cbdcc9e50c89e119736f0/src/vs/editor/browser/viewParts/lines/webgl/base/Color.ts#L270-L275

A color cache where a color always uses the same IColor object would reduce memory usage and creation overhead. It would also prevent possible problems like #4128 from happening.