Initial support for variable width fonts

When translating a logical screen position (columns/rows) to a pixel
position, the editor now builds a temporary version of the line for the
given row. It then uses the DOM range API to insert an empty range
at the correct text node and offset for the given column and determines
the left position based on its clientRect.

Depending on the speed impact, we may want to optimize this by
recycling the existing line node if it exists on screen rather than
building a new one every time. We will still have to build one if the
line we're moving to isn't on screen yet. We could also increase the
chances of the line being on screen by autoscrolling to the vertical
position first, and *then* calculating the horizontal position. Lots
to explore here.

src/app/editor.coffee

@@ -1184,8 +1184,31 @@ class Editor extends View
     @pixelPositionForScreenPosition(@screenPositionForBufferPosition(position))
 
   pixelPositionForScreenPosition: (position) ->
-    position = Point.fromObject(position)
-    { top: position.row * @lineHeight, left: position.column * @charWidth }
+    {row, column} = Point.fromObject(position)
+    [lineElement] = @buildLineElementsForScreenRows(row, row)
+    @renderedLines.append(lineElement)
+    left = @positionLeftForLineAndColumn(lineElement, column)
+    @renderedLines[0].removeChild(lineElement)
+    { top: row * @lineHeight, left: left }
+
+  positionLeftForLineAndColumn: (lineElement, column) ->
+    return 0 if column is 0
+
+    delta = 0
+    iterator = document.createNodeIterator(lineElement, NodeFilter.SHOW_TEXT, acceptNode: -> NodeFilter.FILTER_ACCEPT)
+    while textNode = iterator.nextNode()
+      nextDelta = delta + textNode.textContent.length
+      if nextDelta >= column
+        offset = column - delta
+        break
+      delta = nextDelta
+
+    range = document.createRange()
+    range.setEnd(textNode, offset)
+    range.collapse()
+    leftPixels = range.getClientRects()[0].left - @scrollView.offset().left + @scrollView.scrollLeft()
+    range.detach()
+    leftPixels
 
   pixelOffsetForScreenPosition: (position) ->
     {top, left} = @pixelPositionForScreenPosition(position)