Crash in vtkRenderer::Render() when backing store is on

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

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If you activate the backing store (by calling vtkRenderer::BackingStoreOn()), and then quickly enlarge the window, the application will sometimes crash with a segmentation fault.

To reproduce the bug, you can use the attached tcl file (a modified version of Cone5.tcl). Grab the right side of the window and quickly move it left-right. If you are (un)lucky the application will crash.

I tracked the bug down to the vtkRenderer::Render() function. It does a series of checks to see whether the backing store is still valid. It doesn't check whether the size of the window has changed. It then copies the data from the backing store to the window, using the current size of the window. If the window size is bigger, it will copy more data than what was stored in the buffer. Depending on what is in memory after the buffer, this results in a crash, or just some garbage on the screen (that is cleared by the next render).

I'm attaching a simple patch that fixes the problem. It stores the previous size of the render window and uses the backing store only if the size hasn't changed.