More rotation WIP

Busy with work but managed a few more updates to the rotation.

The aliens are drawn now in the correct position and orientation, as is the bottom line of the screen, the remaining bases icons and the player is sort-of but not-quite right.

As yet I haven't had to change many constants at all; most values are calculated from the original with a macro, which is nice. The main changes have been to the sprite erasing/rendering routines, for which there are about a half-dozen. These are temporary hacks until I optimise the sprite data format.

There will be a few instances where I need to patch the code so that it differs slightly from the original. For example, the saucer sprite is 24 pixels wide, with 4 zero pixels each side to negate the need for erasing it as it moves. However, the Score Advance Table draws only the middle 16 pixels; something I can't do on a rotated screen. For that matter the saucer render will have to call the shifted sprite routine, rather than the simple (non-shifted) sprite routine.

Anyway, coming along, if a little more involved than I first envisaged.