tmxrasterizer: Default to 'png' when target format not recognized

Normally the format is derived from the file name, but that doesn't work
for the temporary files used when tmxrasterizer is called as
thumbnailer.

src/tmxrasterizer/tmxrasterizer.cpp

@@ -159,6 +159,10 @@ int TmxRasterizer::render(const QString &mapFileName,
 
     // Save image
     QImageWriter imageWriter(imageFileName);
+
+    if (!imageWriter.canWrite())
+        imageWriter.setFormat("png");
+
     if (!imageWriter.write(image)) {
         qWarning().nospace() << "Error while writing " << imageFileName << ": "
                              << qPrintable(imageWriter.errorString());