minui: fix screen update issue

yres_virtual value is set incorrectly, causing serveral images to be skipped.
Change this value according to the number of buffers to fix this issue.

from: codeaurora.org
minui: Display the battery charging image correctly
commit: 581a4dead6b96579a13ff22e2454c1f329731679

Change-Id: I10f5d1c6cc37705f0287c7dd517082de2e11d264
Signed-off-by: Iliyan Malchev <malchev@google.com>

minui/graphics.c

@@ -44,6 +44,8 @@
 #define PIXEL_SIZE   2
 #endif
 
+#define NUM_BUFFERS 2
+
 typedef struct {
     GGLSurface texture;
     unsigned cwidth;
@@ -54,7 +56,7 @@ typedef struct {
 static GRFont *gr_font = 0;
 static GGLContext *gr_context = 0;
 static GGLSurface gr_font_texture;
-static GGLSurface gr_framebuffer[2];
+static GGLSurface gr_framebuffer[NUM_BUFFERS];
 static GGLSurface gr_mem_surface;
 static unsigned gr_active_fb = 0;
 
@@ -162,7 +164,7 @@ static void get_memory_surface(GGLSurface* ms) {
 static void set_active_framebuffer(unsigned n)
 {
     if (n > 1) return;
-    vi.yres_virtual = vi.yres * PIXEL_SIZE;
+    vi.yres_virtual = vi.yres * NUM_BUFFERS;
     vi.yoffset = n * vi.yres;
     vi.bits_per_pixel = PIXEL_SIZE * 8;
     if (ioctl(gr_fb_fd, FBIOPUT_VSCREENINFO, &vi) < 0) {