Scroll x 2 pixels, 4 pixels and 8 pixels (char level)

[boriel]

A general purpose scroll routine has been planned for ages, but I also lack time
In general, one like the current already provided, but allowing to specify the number of pixels to scroll in a single call.
However, this will require time to be tested, and I still need to work on other areas of the compiler (currently arrays, but later support for 128K models, etc).