Better error message for Program.
Just show the message in the exception, not print it;
otherwise the user will have to scroll to above the exception trace.
Always "upload" attributes (no early exit if not dirty)
It seems that (at leas on my machine) attributes are *not*
local to the Program object. To test this, run the splitscreen
example.
Adding split-screen example.
Demonstrates two viewports that each have a different visualization.
Resize and mouse events are propagated from the canvas to the viewports.
The scenegraph that we will implement will probably do something similar
(but more advanced :))
See issue #36