Fixed several examples

- Fix line - need to set to agg since gl is default
- Fix Line visual - agg always uses antialias, don't import
  pyopengl unless we need it
- Rename and fix gridlines.py and thus fix the grid demo
- Fix imshow_cuts demo, also fixed that variable.set_data needs
  to call buffer.set_data instead of buffer[...] = data
- Turn blending on in custom_visual.py

the camera.py example I cannot test, since I don't have opencv
installed, but I cannot imagine it having to do with vispy.

Ready for review/merge