Scroll acceleration

[loganzartman]

Both Unix (VT-like terminal) and Windows support scrolling sub-regions of the terminal's buffer contents. Add a method to Screen such as Screen.scroll_buffer() which accepts a rectangle and a scroll vector. This will:

1. scroll the contents of the Screen's buffer, marking each cell as not dirty
2. force an update
3. set the backend's scroll region and ask the backend to perform the scroll

If the backend does not support scrolling, the Screen will instead mark the scrolled cells as dirty and not perform steps 2 or 3.