ui/cocoa: Redraw at correct size when switching surface

If the surface switch involved a resize, we were doing the redraw
at the old size rather than the new, because the update of
screen.width and screen.height was being done after the setFrame
method calls which triggered a redraw. Normally this isn't very
noticeable because typically after the guest triggers the window
resize it also draws something to it, which will in turn cause
us to redraw. However, the combination of a guest which never
draws to the display and a command line setting of a screen size
larger than the default can reveal odd effects.

Move most of the handling of resizes to the top of the method,
and guard it with a check that the surface size actually changed,
to avoid unnecessary operations (including some user visible ones
like "recenter the window on the screen") if the surface is the
same size as the old one.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1387853507-26298-2-git-send-email-peter.maydell@linaro.org
