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
There are some situations when antialiasing in the fragment shader gives incorrect results (for example, this is apparent in the cloud demo when the points are not drawn back to front). In this case we can get better results using FSAA, but only if agg-aa is disabled.