Multi-colour text and graphics closer to working

Running bouldermark I realised that multicolour text and graphics modes had some major problems. 

So I revisited the VHDL code for them, found that multi-colour graphics was simply not implemented at all, and multi-colour text mode had some big fat bugs.

A bit of poking around and sorting things out has it much better, although far from perfect.  For example, some games seem to be largely working now:

And bouldermark, while still not perfect, has the right colours for the most part, although one of the multi-colour bit combinations is pulling the colour from the wrong place, as can be seen by the bricks being mostly invisible in the last image.  But it is progress nonetheless.