gtk3: Avoid crash with some themes (unico based)

We didn't check wether the window was big enough to draw all the widgets. Sometimes, when the window was displaying, it was too small and drawing outside of the window caused a crash, we didn't take into account that the root point for drawing gtk3 widgets wasn't always (0,0).

vcl/unx/gtk3/gdi/gtk3salnativewidgets-gtk.cxx

@@ -1002,8 +1002,10 @@ void GtkSalGraphics::renderAreaToPix( cairo_t *cr,
 
     unsigned char *src = data.get();
     src += (int)ay * nStride + (int)ax * 3;
+    awidth = MIN (region->width, size.getX() - ax);
+    aheight = MIN (region->height, size.getY() - ay);
 
-    for (int y = 0; y < size.getY(); ++y)
+    for (int y = 0; y < aheight; ++y)
     {
         for (int x = 0; x < awidth && y < aheight; ++x)
         {