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.

src/gutter-component.coffee

@@ -16,12 +16,16 @@ GutterComponent = React.createClass
   measuredWidth: null
 
   render: ->
-    {scrollHeight, scrollViewHeight, onMouseDown} = @props
+    {scrollHeight, scrollViewHeight, onMouseDown, backgroundColor, gutterBackgroundColor} = @props
+
+    if gutterBackgroundColor isnt 'rbga(0, 0, 0, 0)'
+      backgroundColor = gutterBackgroundColor
 
     div className: 'gutter', onClick: @onClick, onMouseDown: onMouseDown,
       div className: 'line-numbers', ref: 'lineNumbers', style:
         height: Math.max(scrollHeight, scrollViewHeight)
         WebkitTransform: @getTransform()
+        backgroundColor: backgroundColor
 
   getTransform: ->
     {scrollTop, useHardwareAcceleration} = @props
@@ -47,7 +51,7 @@ GutterComponent = React.createClass
   shouldComponentUpdate: (newProps) ->
     return true unless isEqualForProperties(newProps, @props,
       'renderedRowRange', 'scrollTop', 'lineHeightInPixels', 'mouseWheelScreenRow', 'lineDecorations',
-      'scrollViewHeight', 'useHardwareAcceleration'
+      'scrollViewHeight', 'useHardwareAcceleration', 'backgroundColor', 'gutterBackgroundColor'
     )
 
     {renderedRowRange, pendingChanges, lineDecorations} = newProps