Rotation

Started on the rotation tonight. There is a build option to specify rotated or non-rotated screen, mainly for prosperity. To this end, I've defined a couple of macros that calculate the correct VRAM address (at assembly time) depending on the rotation option, so the code is still clean.

Rotation of 8 & 16-pixel-wide graphics done

It's very preliminary atm. I've only rotated 8x8 pixel blocks; although sprites that are wider can be rendered (see above) they really should be reorganised to optimise rendering. That'll come later.

I've retained the 256-pixel-wide screen for now, to simplify the rotation. When it's all done, I'll change it to 320 pixels wide (hopefully just modify a macro or two) and move the scores.