spice: surface switch fast path requires same format too.

Commit "555e72f spice: rework mirror allocation, add no-resize fast path"
adds a fast path for surface switches which does't go through the full
primary surface destroy and re-recreation in case the new surface is
identical to the old one (page-flip).  It checks the size only though,
but the format must be identical too.  This patch adds the format check.

Commit "0002a51 ui/spice: Support shared surface for most pixman
formats" increases the chance to actually trigger this.

https://bugzilla.redhat.com/show_bug.cgi?id=1247479

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
