fixed #3 by moving the call to make curront to the top of snapshot

function to prevent the push calls being made without a valid context or
a different context than the pop calls.

widgets/glWidget/QtBaseViewer.cc

@@ -2200,7 +2200,8 @@ void glViewer::slotPropertiesUpdated() {
 }
 
 void glViewer::snapshot(QImage& _image, int _width, int _height, bool _alpha, bool _hideCoordsys, int samples) {
-    
+    makeCurrent();
+
     int w = 0, h = 0, bak_w = 0, bak_h = 0, left = 0, bottom = 0;
     
     glstate_->push_modelview_matrix();
@@ -2240,7 +2241,6 @@ void glViewer::snapshot(QImage& _image, int _width, int _height, bool _alpha, bo
     // it will fall back to the maximal supported number of samples
     format.setSamples(samples);
 
-    makeCurrent();
     QFramebufferObject fb(w,h,format);
 
     if ( fb.isValid() ){