Fix variable mixup from e8f64be6

Just got this warning:

../OpenFlipper/libs_required/ACG/GL/globjects.cc:643:51: warning: variable 'gltex' is uninitialized when used within its own initialization [-Wuninitialized]
      QImage gltex = ACG::Util::convertToGLFormat(gltex);

Introduced by a recent commit that mixed up two variables:

-      QImage gltex = QGLWidget::convertToGLFormat ( qtex );
+      QImage gltex = ACG::Util::convertToGLFormat(gltex);

I didn't test this change, but it seems clear enough that change was unintended.

libs_required/ACG/GL/globjects.cc

@@ -640,7 +640,7 @@ bool Texture2D::loadFromFile( const std::string& _filename, GLenum _minFilter, G
       if (mipmaps)
         autogenerateMipMaps();
 
-      QImage gltex = ACG::Util::convertToGLFormat(gltex);
+      QImage gltex = ACG::Util::convertToGLFormat(qtex);
 
       setData(0, GL_RGBA, gltex.width(), gltex.height(), GL_RGBA, GL_UNSIGNED_BYTE, gltex.bits());
     }