Eliminate ScreenLine.bufferRows property

Previously, we used bufferRows to determine how many buffer rows were
spanned by the screen line in the event it was folded. Now that we have
RowMap, screen lines don't need to serve this purpose. We still need
to know if a screen line is wrapped, which we solve via setting the
lineEnd property to null for a soft-wrapped screen line.

src/app/screen-line.coffee

@@ -4,14 +4,13 @@ _ = require 'underscore'
 
 module.exports =
 class ScreenLine
-  constructor: ({tokens, @lineEnding, @ruleStack, @bufferRows, @startBufferColumn, @fold, tabLength}) ->
+  constructor: ({tokens, @lineEnding, @ruleStack, @startBufferColumn, @fold, tabLength}) ->
     @tokens = @breakOutAtomicTokens(tokens, tabLength)
-    @bufferRows ?= 1
     @startBufferColumn ?= 0
     @text = _.pluck(@tokens, 'value').join('')
 
   copy: ->
-    new ScreenLine({@tokens, @ruleStack, @bufferRows, @startBufferColumn, @fold})
+    new ScreenLine({@tokens, @ruleStack, @startBufferColumn, @fold, @lineEnding})
 
   clipScreenColumn: (column, options={}) ->
     return 0 if @tokens.length == 0
@@ -75,10 +74,9 @@ class ScreenLine
 
     leftFragment = new ScreenLine(
       tokens: leftTokens
-      bufferRows: 0
       startBufferColumn: @startBufferColumn
       ruleStack: @ruleStack
-      lineEnding: @lineEnding
+      lineEnding: null
     )
     rightFragment = new ScreenLine(
       tokens: rightTokens
@@ -89,7 +87,7 @@ class ScreenLine
     [leftFragment, rightFragment]
 
   isSoftWrapped: ->
-    @bufferRows == 0
+    @lineEnding is null
 
   tokenAtBufferColumn: (bufferColumn) ->
     delta = 0