Clip to next boundary when seeking iterator to the middle of a text tag

Previously, when calling `TokenizedBufferIterator.seek` with a position
that lied within a text tag, we advanced the iterator by the extent of
that tag without, however, consuming it. Hence, when calling
`moveToSuccessor` afterward, we would consume that tag and advance the
iterator again, thus effectively moving it twice and making its position
inaccurate.

An option could be to clip to the left of the textual tag without
consuming it. However, this would be a little odd with respect to the
current contract between (`DisplayLayer` and) `seek`, whose promise is
to move the iterator to a position that is greater or equal than the one
asked by the caller.

Therefore, with this commit, we are changing the behavior of `seek` in
this particular scenario to consume the tag in question and process all
its siblings until a tag boundary is finally found. This ensures that
the above contract is always respected, while still preserving the "seek
to leftmost tag boundary" semantics (i.e. notice how in the changed test
case, calling `seek` with `Point(0, 1)` is the same as calling it with
`Point(0, 3)`).

src/tokenized-buffer-iterator.coffee

@@ -21,21 +21,18 @@ class TokenizedBufferIterator
 
     for tag, index in @currentTags
       if tag >= 0
-        if currentColumn is position.column
+        if currentColumn >= position.column
           @tagIndex = index
           break
         else
           currentColumn += tag
           @containingTags.pop() while @closeTags.shift()
           @containingTags.push(openTag) while openTag = @openTags.shift()
-          if currentColumn > position.column
-            @tagIndex = index
-            break
       else
         scopeName = @tokenizedBuffer.grammar.scopeForId(tag)
         if tag % 2 is 0 # close tag
           if @openTags.length > 0
-            if currentColumn is position.column
+            if currentColumn >= position.column
               @tagIndex = index
               break
             else