Account for scaling of surfaceInset area in magnification.

We don't want to scale the actual size of the surface as we
were doing prior to ag/1101405 but we do need to adjust the top
and left corners so that the content will be at the same position
despite the inset area being scaled up.

Bug: 30399709
Change-Id: I30df4d3ef593d4f2869fdf378ee9bc3469619d5d