Scrollable Canvas

[WillGreen]

Allow display area of bitmap canvas to be scrolled. Needs to support wrapping. Will require changes to Canvas Display AGU.

This is really two issues:

1. Vertical scrolling - updating starting address is enough if we can wrap correctly at the of buffer
2. Horizontal scrolling - need to wrap around to start of line and handle pixels being packed into word

Canvas Display AGU doesn't use multiplication and we want to avoid adding multiplication to support scrolling.