Let fbsplash display images even when a second buffer is active

If a previous application uses double buffering and makes the
second buffer visible, fbsplash's images (written to the first
buffer) otherwise won't be visible.

Signed-off-by: Dan Fandrich <dan@coneharvesters.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

miscutils/fbsplash.c

@@ -83,11 +83,14 @@ static void fb_open(const char *strfb_device)
 
 	// map the device in memory
 	G.addr = mmap(NULL,
-			G.scr_var.xres * G.scr_var.yres
+			G.scr_var.xres * G.scr_var.yres_virtual
 			* BYTES_PER_PIXEL /*(G.scr_var.bits_per_pixel / 8)*/,
 			PROT_WRITE, MAP_SHARED, fbfd, 0);
 	if (G.addr == MAP_FAILED)
 		bb_perror_msg_and_die("mmap");
+
+	// point to the start of the visible screen
+	G.addr += G.scr_var.yoffset * G.scr_fix.line_length + G.scr_var.xoffset * BYTES_PER_PIXEL;
 	close(fbfd);
 }