[client] video: switch transport sources

Track primary and fallback video with source-specific generations, and
publish a source only after its first frame or software surface is
ready.

Route SPICE surfaces and cursors through the generic render queue. Keep
the current image across handoffs, and switch input only after the new
source has been presented.

Move fallback activation to its worker so display callbacks cannot
block or deadlock the render path.

client/tests/mouse_test.c

@@ -262,6 +262,11 @@ bool app_isRunning(void)
   return false;
 }
 
+void app_invalidateWindow(bool full)
+{
+  (void)full;
+}
+
 uint64_t app_mouseSeq(void)
 {
   return 0;