display on linux/gtk

 - RenderTexture unbinds itself on frame buffer configuration fail,
   before raising an error.
 - RenderTexture was potentially doing integer division on texture
   size ratio calculation
 - GL21 LineVector software draw could possibly be called before
   line vertices had been created.
 - Line/RGB vector command line args were not enabled
 - RGBVector was not initialising texture interpolation correctly

SimpleMessageDialog, which just displays a plain message, and
TimeoutDialog, which shows said message, and disappears after a
timeout. ProcessingDialog now inherits form SimpleMessageDialog.

Another dialog, TextEditDialog, shows an editable/selectable piece of
text. This is used by a new CanvasPanel action, showCommandLineArgs,
which pops up a dialog allowing the user to copy the command line to
recreate the currently visible scene.