TgaImporter: quick ugly fix - don't delete the image data on closing.

It causes problems in ColladaImporter, because it uses TgaImporter for
opening the image, then takes ownership of the data and then destroys
the importer. Need to think this thoroughly.

src/Plugins/TgaImporter/TgaImporter.cpp

@@ -96,7 +96,8 @@ bool TgaImporter::open(istream& in, const string& name) {
 }
 
 void TgaImporter::close() {
-    delete _image;
+    /** @todo fixme: delete it only if it wasn't retrieved by user */
+    //delete _image;
     _image = nullptr;
 }