Support opaque selections in DOM renderer

[Tyriar]

Brings the DOM renderer closer to the webgl renderer which draws the text
on top of the selection. This is done by leveraging the override system
similar to in webgl and as well as forcing the element above the selection.
This also improves contrast color caching for the DOM renderer.

This should result in a slight performance hit but only when a selection
is being made. The improved contrast seems worth it.

Fixes #3838

---

term.setOption('rendererType', 'dom');
term.setOption('minimumContrastRatio', 5);
term.setOption('theme', {
    selection: '#ffffff',
    selectionForeground: '#000000'
});

Before:

After:

---

Using this to show minimum contrast ratio is working:

term.setOption('rendererType', 'dom');
term.setOption('minimumContrastRatio', 5);
term.setOption('theme', {
    selection: '#ff0000',
    foreground: '#ff1111'
});