[Fix] Fixed thumbnail only generating last page.

src/thumbgen.cpp

@@ -26,7 +26,7 @@ void ThumbGen::run(void) {
     mutex.lock();
     PDFPageWidget* pw = pWidgets.first();
     pWidgets.pop_front();
-    Poppler::Page* pp = pPages.last();
+    Poppler::Page* pp = pPages.first();
     pPages.pop_front();
     mutex.unlock();