Make mythgallery's thumb generator not auto-detect

The QImageReader autodetection seems to be overkill for what we need here.  By
turning this off, only registered image extensions will be tried.  This will
significantly speed up scanning directories with mixed content in them.

Hopefully, this will not adversely affect "normal" directories of only images.

Fixes #9523

Signed-off-by: Gavin Hurlbut <ghurlbut@mythtv.org>

Author: Lawrence Rust

mythplugins/mythgallery/mythgallery/thumbgenerator.cpp

@@ -243,6 +243,7 @@ void ThumbGenerator::loadDir(QImage& image, const QFileInfo& fi)
     {
         f = &(*it);
         QImageReader testread(f->absoluteFilePath());
+        testread.setAutoDetectImageFormat(false);
         if (testread.canRead())
         {
             found = true;