Measure the rendered char width instead of the buffer char

If invisibles are turned on, the buffer char and the rendered char
may differ in size. For example, if `☃` is used as the tab invisible 
its width is probably larger than the width of a `\t`

Closes #1724

src/editor-view.coffee

@@ -1338,14 +1338,17 @@ class EditorView extends View
     return 0 if screenColumn == 0
 
     tokenizedLine = @editor.displayBuffer.lineForRow(screenRow)
+    textContent = lineElement.textContent
 
     left = 0
     index = 0
     for token in tokenizedLine.tokens
-      for char in token.value
+      for bufferChar in token.value
         return left if index >= screenColumn
 
-        val = @getCharacterWidthCache(token.scopes, char)
+        # Invisibles might cause renderedChar to be different than bufferChar
+        renderedChar = textContent[index]
+        val = @getCharacterWidthCache(token.scopes, renderedChar)
         if val?
           left += val
         else