Fix bug where CPU could be stuck spinning

Required the wallpaper be animating and
the system to have run slowly during the
last frame when rendering stopped.

bug 5180240

Change-Id: I38c2b6f07a1e36019dd05c77a58973e36d12728c

libs/rs/rsContext.cpp

@@ -279,6 +279,8 @@ void * Context::threadProc(void *vrsc) {
             rsc->timerSet(RS_TIMER_INTERNAL);
             rsc->timerPrint();
             rsc->timerReset();
+        } else {
+            doWait = true;
         }
     }