:lipstick:
Apply background color of root editor node to lines as an inline style

This ensures lines have an opaque background that matches whatever the
editor is styled as, but avoids the need to apply the .editor-colors
class to the .lines div. That approach fell down when people were
setting the background color via means other than .editor-colors, such
as styling mini editors via the .editor.mini selector in the settings
view.
Give the line-numbers div an opaque background for better GPU perf

We sample both the background color of the editor and the gutter. If the
gutter has an actual background color, we use it. Otherwise we fall back
to the same background as the editor.
Resample background colors whenever stylesheets change

This prevents a flicker that occurred on theme changes when we just
detected this via polling. I'm going to leave the polling as a catch-all
in case the editor is styled inline in some way.
Merge pull request #3077 from atom/ns-fix-scroll-perf-regression

Fix performance regression caused by transparent backgrounds on lines and gutter