improve scroll region function

[stevesims]

fixes scrolling background colour issue - the new area of screen will now always be filled with the current text background colour

also adds support for movement directions 4-7, which move dependent upon the currently set cursor behaviour bits (see VDU 23,16)

adds support for movement amount of 0, to indicate move by 1 character in the given direction (as per Acorn’s implementation)

fixes #58