Enable GL2PS export for OpenGL2 backend.
- Modified gl2ps to work without an OpenGL context (add geometry manually).
- Use OpenGL TransformFeedback buffers to capture vertex data (replace deprecated "glRenderMode(GL_FEEDBACK)")
- Add export for:
- vtkTextActor, vtkTextActor3D, vtkTextMapper
- All Context2D stuff (except point sprites)
- All other geometry is rasterized into the exported file's background.