Add a priority constant in to the graphics HAL header

this is needed if a display/gpu driver needs to create
service threads that can block the main ui thread.
to avoid priority inversions, these must run at
HAL_PRIORITY_URGENT_DISPLAY

Change-Id: I3af592e05a6d1b6f39a2b7885a7c842e54abfce2

include/system/graphics.h

@@ -19,6 +19,20 @@
 
 __BEGIN_DECLS
 
+/*
+ * If the HAL needs to create service threads to handle graphics related
+ * tasks, these threads need to run at HAL_PRIORITY_URGENT_DISPLAY priority
+ * if they can block the main rendering thread in any way.
+ *
+ * the priority of the current thread can be set with:
+ *
+ *      #include <sys/resource.h>
+ *      setpriority(PRIO_PROCESS, 0, HAL_PRIORITY_URGENT_DISPLAY);
+ *
+ */
+
+#define HAL_PRIORITY_URGENT_DISPLAY     (-8)
+
 /**
  * pixel format definitions
  */