add optional overscan compensation to recovery graphics

If your screen is a TV, it may not actually be displaying the edges of
the framebuffer.  Allow specifying an overscan percentage, and move
each edge of the framebuffer in by that percent of the width/height.
(The gr_* layer just lies to the caller about the size of the
framebuffer, telling the caller it's smaller than it really is, and
offsets all drawing commands to match.)

Change-Id: I11bb2feb39ae522bd3e957a14ebdecf3609e0fdc