GlslEditor: Fix crash in GlslHighlighter

since the async highlighter implementation the parent is not a
TextDocument anymore, but a QTextDocument. Pass the required mimeType
via the SyntaxHighlighterRunner now to the SyntaxHighlighter.

Change-Id: I0afcbe68719195649b4b7040416a743cb01214df
Reviewed-by: Artem Sokolovskii <artem.sokolovskii@qt.io>

src/plugins/texteditor/syntaxhighlighter.cpp

@@ -61,6 +61,7 @@ public:
     QList<std::pair<int,TextStyle>> formatCategories;
     QTextCharFormat whitespaceFormat;
     bool noAutomaticHighlighting = false;
+    QString mimeType;
 };
 
 static bool adjustRange(QTextLayout::FormatRange &range, int from, int charsDelta)
@@ -364,6 +365,18 @@ QTextDocument *SyntaxHighlighter::document() const
     return d->doc;
 }
 
+void SyntaxHighlighter::setMimeType(const QString &mimeType)
+{
+    Q_D(SyntaxHighlighter);
+    d->mimeType = mimeType;
+}
+
+QString SyntaxHighlighter::mimeType() const
+{
+    Q_D(const SyntaxHighlighter);
+    return d->mimeType;
+}
+
 /*!
     \since 4.2