Add A8 surface capability

Even though the ability to handle a8 surfaces was added at the same
time as the composite command, they are logically separate, so add a
capability bit to indicate the presence of a8 surfaces.

spice/protocol.h

@@ -128,6 +128,7 @@ enum {
     SPICE_DISPLAY_CAP_SIZED_STREAM,
     SPICE_DISPLAY_CAP_MONITORS_CONFIG,
     SPICE_DISPLAY_CAP_COMPOSITE,
+    SPICE_DISPLAY_CAP_A8_SURFACE,
 };
 
 enum {