Fix highlighting of C/C++ line continuation backslashes

The backslash itself is not a token, so we must make sure to only format
the trailing whitespace characters as visual whitespace, instead of
everything trailing.

Broke in a92694596e

Task-number: QTCREATORBUG-987
Change-Id: I4e9e84d29513ea317d7e3bde6c6b4c43749cb649
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/syntaxhighlighter.h

@@ -33,6 +33,7 @@
 #include <QTextLayout>
 
 #include <functional>
+#include <limits.h>
 
 QT_BEGIN_NAMESPACE
 class QTextDocument;
@@ -84,7 +85,7 @@ protected:
     void setFormat(int start, int count, const QFont &font);
     QTextCharFormat format(int pos) const;
 
-    void formatSpaces(const QString &text);
+    void formatSpaces(const QString &text, int start = 0, int count = INT_MAX);
     void setFormatWithSpaces(const QString &text, int start, int count,
                              const QTextCharFormat &format);