Revert "Render highlights on their own layer to avoid GPU artifacts"
Render a dummy highlight decoration to prevent artifacts

When the last highlight gets removed, we get a rendering artifact.
Always rendering an empty dummy at 0,0 ensures that never happens.
Merge pull request #2708 from atom/revert-2699-ns-react-fix-artifacts

Avoid GPU artifacts without rendering selections on another layer
Style width of lines inline so they're always full width

This is important so that styles applied to lines extend all the way
across the editor. I tried to use min-width, but it was causing a
full-screen repaint during scroll, which setting the width explicitly
doesn't do.
WIP: Only render cursor-line decoration on lines with empty markers