text: Title: ExportDialog Drawn Off Screen
  Body: Depending on the size of the scene the export dialog box can be drawn off or partially off screen. This is due to an implementation of the `show` command that allows moving the box to negative pixel indices.
  
  Problem Code:
  https://github.com/pyqtgraph/pyqtgraph/blob/a5f48ec5b58a10260195f1424309f7374a85ece7/pyqtgraph/GraphicsScene/exportDialog.py#L57-L62
  
  To fix this, the position calculation can be clipped using `max`, and the `setGeometry` command can be changed to `move` to account for the size of the window's frame.
  
  Potential Fix:
  ```python
          if not self.shown:
              self.shown = True
              vcenter = self.scene.getViewWidget().geometry().center()
              x = max(0, int(vcenter.x() - self.width() / 2))
              y = max(0, int(vcenter.y() - self.height() / 2))
              self.move(x, y)
  ```
  
  I can't say I understand the motivation for moving the dialog box in the first place, but atleast with this modification the dialog box is always accessible with the mouse.