Render highlights on their own layer to avoid GPU artifacts

Previously, when the last highlight div was removed from the lines
layer, chunks of the lines would sometimes disappear. Since we've
discovered that giving the lines an opaque background doesn't help with
subpixel anti-aliasing anyway, I've found that rendering highlights on
their own layer behind the lines and making the lines layer transparent
avoids the arficacts.
Don't render an opaque background behind line numbers

It doesn't help subpixel anti-aliasing like I thought, so screw it.
Add a dedicated underlayer to avoid GPU artifacts on wrap guide etc

Trying to make the .highlights layer double as the .underlayer was
causing GPU artifacts on the wrap guide when the last highlight was
removed. This puts it in its own layer to avoid that.
Merge pull request #2699 from atom/ns-react-fix-artifacts

Render highlights on their own layer to avoid GPU artifacts