ui/cocoa: Use the pixman image directly in switchSurface

ui/cocoa: Use the pixman image directly in switchSurface

Currently the switchSurface method takes a DisplaySurface. We want
to change our DisplayChangeListener's dpy_gfx_switch callback
to do this work asynchronously on a different thread. The caller
of the switch callback will free the old DisplaySurface
immediately the callback returns, so to ensure that the
other thread doesn't access freed data we need to switch
to using the underlying pixman image instead. The pixman
image is reference counted, so we will be able to take
a reference to it to avoid it vanishing too early.

In this commit we only change the switchSurface method
to take a pixman image, and keep the flow of control
synchronous for now.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-id: 20190225102433.22401-3-peter.maydell@linaro.org
Message-id: 20190214102816.3393-3-peter.maydell@linaro.org