[vtkusers] OSMESA causes some segfaults

[David Doria]

If I compile with VTK_OPENGL_HAS_OSMESA ON, then try to do something
normal (non offscreen rendering) and simple like this:

http://www.cmake.org/Wiki/VTK_Examples_Display_Cube

I get a segfault. It happens in

void vtkXOpenGLRenderWindow::Start(void)

The only reason I turned on HAS_OSMESA was to test the offscreen
rendering example - but shouldn't I be able to keep this on and do
on-screen rendering too?

Thanks,

David