Mark insets with invalid buffer() in red in devel-mode.

Mark insets with invalid buffer() in red in devel-mode.

We tend to have insets which buffer() member is invalid. To help
debugging, this commit paints their background in red when devel-mode
is on.

To this end, a new method develMode() is added to the Painter class.

With this commit, it is easy to see that macro template do not have a
proper buffer set!