Allow disabling page-flipping.

Some devices have low-level bugs in their page-flip.

Disabling it at this level gives us more flexibility
than trying to control it elsewhere.

Bug: 3097357
Change-Id: Icc6ae172264e453bec0680e74a497a2b24299c4a
diff --git a/modules/gralloc/framebuffer.cpp b/modules/gralloc/framebuffer.cpp
index 0195103..a487b5d 100644
--- a/modules/gralloc/framebuffer.cpp
+++ b/modules/gralloc/framebuffer.cpp
@@ -43,7 +43,12 @@
 /*****************************************************************************/
 
 // numbers of buffers for page flipping
+#if defined(NO_PAGE_FLIPPING)
+// page-flipping is buggy on some devices
+#define NUM_BUFFERS 1
+#else
 #define NUM_BUFFERS 2
+#endif
 
 
 enum {