Give lines and line numbers an opaque background to support sub-pixel AA

Since lines and line numbers are now on the GPU, their text won't be
properly anti-aliased on low-resolution displays unless their layers
have a solid background.

src/gutter-component.coffee

@@ -77,7 +77,7 @@ LineNumberComponent = React.createClass
   render: ->
     {index, lineHeight, scrollOffset} = @props
     div
-      className: "line-number"
+      className: "line-number editor-colors"
       style: {WebkitTransform: "translate3d(0px, #{index * lineHeight + scrollOffset}px, 0px)"}
       dangerouslySetInnerHTML: {__html: @buildInnerHTML()}
 

src/lines-component.coffee

@@ -112,7 +112,7 @@ LineComponent = React.createClass
     left = horizontalScrollOffset
     style = WebkitTransform: "translate3d(#{left}px, #{top}px, 0px)"
 
-    div className: 'line', style: style, 'data-screen-row': screenRow, dangerouslySetInnerHTML: {__html: @buildInnerHTML()}
+    div className: 'line editor-colors', style: style, 'data-screen-row': screenRow, dangerouslySetInnerHTML: {__html: @buildInnerHTML()}
 
   buildInnerHTML: ->
     if @props.tokenizedLine.text.length is 0