Framebuffer: Support variable number of framebuffers in the UI

This change defines a macro NUM_FRAME_BUFFERS to set
the desired number of framebuffers to be used by the UI,
instead of hard-coding 2 framebuffers.

Aditional logic has been  added to handle the initialization
and destruction of NUM_FRAME_BUFFERS buffers.

Change-Id: I3a4bfec3e0f453432f2ffebf084c00f574d3be46
Signed-off-by: Rodrigo Obregon <robregon@ti.com>
diff --git a/include/ui/FramebufferNativeWindow.h b/include/ui/FramebufferNativeWindow.h
index 8ea3ab9..e9c7df2 100644
--- a/include/ui/FramebufferNativeWindow.h
+++ b/include/ui/FramebufferNativeWindow.h
@@ -29,6 +29,7 @@
 
 #include <ui/egl/android_natives.h>
 
+#define NUM_FRAME_BUFFERS  2
 
 extern "C" EGLNativeWindowType android_createDisplaySurface(void);
 
@@ -69,7 +70,7 @@
     framebuffer_device_t* fbDev;
     alloc_device_t* grDev;
 
-    sp<NativeBuffer> buffers[2];
+    sp<NativeBuffer> buffers[NUM_FRAME_BUFFERS];
     sp<NativeBuffer> front;
     
     mutable Mutex mutex;