First pass at adding graphics mode

Replaced ‘inverse’ and ‘on’ bools with graphics mode, an enum which
will allow more flexibility in overlaying, negating,  and generally
toggling both characters and graphic primitives.

There are still some problems to resolve:
1. drawString puts bars beside
2. setPixel bit logic is almost certainly banjaxed
3. No tests. sigh. Will add in to the examples.
4. OR on characters didn’t do what I wanted. It appeared to do nothing,
possibly related to point 2.